Trying to keep this brief!

I have 2 of my own children and two step children. I really can't take my sd much longer. Ss not great but is growing out of it now.

I had planned to step back take my two children out all day when step kids here and keep them apart without making it obvious (sk bad influence on my two as they have slack rules with bm and my partner won't give real boundaries here because bm doesn't- common problem I'm sure you all know what I'm talking about). Anyway then covid happens and I'm stuck in the house watching my 5 year old mimic her ss and hero worship her. I get comments like school is stupid and healthy food is disgusting etc all stuf she never believed before sd told her that. 
 

im Aspergers and have a very sensitive sense of smell. Sd makes me nauseous. She stinks because she doesn't wash. I raised it so many time with my partner and he tells her to shower but she goes in there and comes out still stinking just as bad and partner won't tell her to wash properly I think he's too embarrassed. This child is 9 weighs more than me a as an adult! Body odour, matted hair. It's borderline neglect and I've had rows with partner over it. He finally did address it with his ex and she says things are Brian's he parrots that back at me. But she still stinks. She is aware I don't like her I think. She used to like me lots but I started disengagement and she has identified me as the cause of the rules she doesn't like here etc. 

my young son is also autistic and gets challenging behaviour when triggered by step kids. In particular sd smell and the screen time they are allowed. Of course it's not seen as causing sk it's seen as my son. 
 

I don't want my kids around sd. She is a bad influence. I'm irritable and snappy with my own children when step kids here because I spent the whole weekend with the taste of sd bo in my mouth. (I'm not sure she always has bo I think I just hate her smell now) 

BUT ss is growing out of it. He's brilliant with my son, so patient and understanding even when my son is stealing all his stuff. He says hi to me politely where sd will blank me. He's not as bad with his weight as sd and is healthier in general because he's not so badly attitude to veg and exercise etc. He's not doing great at school and his parents are failing him, he used to have anxiety attacks. I just feel like he actually does appreciate me when I talk with him and he is a nice boy just with bad boundaries around screens and age appropriate films etc. 
 

how do you step back from one step child but not the other? It feels like I would be being actively cruel to sd. She's not actively that rude to me she is a typical 9 year old who's parenting has been lacking. I can't say she is so disrespectful I would be fair to say I will tolerate ss but not her. She is not a bad child she's a failed child. But I just don't like her around me and my children I don't want her near me it makes me nauseated and irritated. 
 

any advice? How to I tell my partner I am stepping back from one of his lids but the other one is ok? 
can you even step back from one and not the other? 
im just another adult failing her I know but I am failing my children if I allow her influence to mess them up in the same ways. 
 

failed at breif there haha