I just joined this site in the hope to get some advice. I respect not everyone will have the same opinion usually due to their own personal circumstances. 
 

myself and my pn have been together over two years now I have two children and he has four (one of which he classes as his child but is actually not biologically his) the reason I'm stating that is because she is the only one of them who has had a massive problem with our relationship - Has only seen to pn twice in over 2 years where they've met up for a drink , she gets cross if he mentions myself or my kids etc and has just tried to make things generally difficult for him as has his ex. The ex - is his ex wife they have gotten divorced and it's important that I state he was separated from her when I met him - we didn't have an affair although that's what she's tried to imply and lead her daughter to believe( the one with the problem) . Other children are two with the ex wife and one with a different mother. The one with the different mother visits regularly , she hadn't seen my pn for a year until he got with me apparently hated his ex wife who didn't treat her well and certainly seems to be jealous of her even now which seems to have rubbed off on the other two who pick on her and make her uncomfortable and often smother their dad so she doesn't get a look in . So she has taken to coming around to ours when they're not about and she is an absolute delight . We get on well, she gets on with my children and she's just really sweet , I adore her. The other two however it's been very rocky to say the least. Unfortunately the ex wife has fed them an awful lot of bs and been constantly negative about our relationship even though I've never met her. she has a partner , but constantly tries to cause issues and harasses my pn constantly by text, what's app, email , phone etc with petty things mostly if he doesn't reply straight away or he tells her he can't meet one of her many demands she goes absolutely mental and starts bringing up the past (like I said she moved on straight away) from what I gather she was very possessive towards him and he was the one who separated from her which caused her to hit him quite a few times and desperately attempt to make him stay on multiple occasions. He's absolutely petrified that if he doesn't meet her constant demands she will stop him from seeing his children(13&12) which she previously has threatened numerous times when she doesn't get her own way - through this fear himself and his mother tend to try and meet everything she asks for. We pay her regular maintenance, my pn has them twice one week and 4 times the other week despite them living 15 miles away( a lot of petrol) which she choose to move away to, his mother has them whenever she asks which is usually at least once a week aswell. She then also asks for extra money on top for uniform etc but constantly is out doing lavish things and getting things done for herself(tattoos, weekends away, nights out) we do not have this spare money and she knows it. She purposely organises things for us to take the kids to at the weekends such as surfing and expects us to take them even though it's 20 miles away which then stops us when we've already made plans or is just an absolute pain in the ass especially as when we plan things we do it so all the children can do things together but she's on purposely doing this and telling the kids so they're excited etc. They also refuse to stay at my house and she has insisted that they stay at his mothers house with him at the weekends and not in his home because her kids don't like it. From what I've gathered it's not them that doesn't like it, it's her who encourages them to act out constantly so they can stay at his mothers where they can have what they want and also have their own bedroom with tv/games consoles.  When they come over during the day she calls them and asks them to go away from us so she can privately talk to them. They act very off after they speak with her and when they come during the day for the first few hours they can be horrendous. They'll swear at us or each other ,  fight each other, say rude things about our home and make it clear they don't want to be here. After a few hours they change they start speaking to us and they'll play games with us etc or just watch tv and every time it's the same . We have tried to take them out for walks etc but they refuse to walk and cause a scene and generally act out the whole time and then demand they have a treat as compensation for their stress( which is no longer forfilled) Quite a few times they've come out with things which are clearly not their opinions and their dad has to put them straight. His ex also makes things up that they've said and then when he speaks to the children they say no we didn't say anything like that etc. recently when she had done this one of the children actually said ' she's crazy , she makes things up, I never even said anything like that I don't know why she'd say that' . But they still fiercely protect her of course, she is there mother. It's all very stressful and this is only the literal base of what's going on. But at the moment things are as follows 

-partner has seen non biological daughter from previous marriage(young adult) twice in 2 years . Isn't allowed to mention us but still we buy her cards and presents for birthday and Christmas none of which are allowed to mention any of us in the card or otherwise for fear of rocking the boat with her. 

-we all have a good relationship with first born daughter from other ex and absolutely no issues other than she prefers to come when his other kids aren't here which is fine (young adult)

- has two younger (teenagers) at his mums house over night every other weekend, and twice in the week every week. 
Children refuse to stay at our house, they're rude, and they are allowed to eat whatever they want even if it's a 6 pack of crisps for tea. Not only that but they do not eat anything other than junk food for dinner or otherwise and constantly have an abundance of sweets and only drink fizzy drink. They are very overweight. They have bad manners and bad attitudes. But when my partner enforces rules in our house they will follow after a while but usually takes a few hours.he does not buy them fizzy drinks , energy drinks or sweets but they do get away with only eating junk food for tea. They've never had any rules or boundaries previously.My pn has tried to enforce rules but these end up being undone by his ex and his mother who continue to let the children do and have what they want and make excuses for them. This is because they're mildly autistic and that's always been there reason to make excuses for everything they either don't want to do or don't want to eat or drink  . They do no exercise and they get signed off or by their mother.m because they don't like doing it.  My daughter has adhd but she has always had rules and boundaries and so it's very difficult as she isn't allowed to eat junk food constantly , or anything else that they are used to doing. They don't act like children they spend all their time on games consoles and watching tv this is all I've seen them do in the last 2 years other than occasionally one will join in with a game but not the other. I have never known them to do anything else other than the one time I offered to take them out and once realising I wasn't going to by them junk to sit there and eat they actually interacted with others and looked almost like children for a short time. They don't do anything for themselves they expect others to do everything for them, pick up after them, carry their things etc. I completely understand autism as I used to work with SEN kids but these children use this as an excuse for everything and anything which has been encouraged by their Nan and their mother.
 

My partner works 6 days a week aswell so obviously we are having minimal time together and usually on the time we do get together a drama come up with the ex. In the last two years she has kicked out one of the children twice apparently due to their behaviour and we had them with us both times, once at his mums and the other she asked to come here as my pn said she'd have to come here as he wasn't going to stay at his mums the whole time.she accepted and came here and was completely different, even followed rules.  We also had them for 2 weeks in the summer as she said she needed to go away and although one of them was distraught she refused to see them until the 2 weeks was up and for that period they stayed with my pn at his mums this was precious to the last time I just mentioned as we then decided that it couldn't be like that all the time. We tried having them every other weekend at our house but the mother kicked off so much I that they ended up going back to stay in at nans.

Before this relationship I was single for 7 years and have not had a relationship like this one before. I was a single parent to my children with no involvement from their father. This situation causes a lot of stress which I expected but don't know how to deal with. 
 

 Do I just accept that things will never change and keep them separate from our life together . For example he stays at his mothers every other weekend aswell as the twice in the week he has them and I we only see him a few hours of the other evenings he has left (usually not as he's exhausted or has other things to do or needs to see one of the other children) other than that we have him every other Sunday at home unless some drama crops up with the ex. Every single Sunday we have together she will message some crap or drama without fail for the last 2 years to ruin our bit of time together. 
 

is there any other way to try and battle through it until they're older? I honestly don't know. I advised him ages ago to take her to court but he won't he says he doesn't want to drag his kids through that which I understand but that means himself and his mother have to do everything she wants and it literally is like our lives are being run by this woman.