Me and my boyfriend have been dating for almost 3 years. We entered the relationship with his and mine and now we have the his, mine, and ours. He has a girl who just turned 6. I have a 7 yr old boy and a 6 yr old girl as well. They are a month apart. Together we have a almost 2 year old and a 10 month old. Our relationship started very well all kids were treated equal and accepted. He has his daughter every other weekend and I have my kids 50/50 same days during the week/alternating weekends.
Our relationship started to head down hill and fast. When he would get upset he would put his daughter on a pedostile and parade her around. Not literally, obviously but she wouldn't get in trouble for things that my two would, she would get things and my two wouldn't, my two were under strict rules but none of them applied to his. My kids would even ask why does she when they can't. 
My boyfriends sister lived with us for a short period and talked to me about this behavior that was going on as she was noticing it as well - speaking of her brother and her niece. The sound of her voice was like nails on a chalk board and she literally is the tornado to our weekends, by the time Sunday come around and she left we were all standing there in dismay, hair amess, wondering what the hell just happened. Behaviors include everything big and small, you name it, she did it. 
When our daughter was born, she come parading out of her room one morning telling us about her dream and how she killed the baby. What finally did it for me was my daughter had a baby, which was hers, his daughter came and took it from her, my daughter went to her room and cried, I was so frustrated with the situation, I told my daughter to find something else to play with. Dad heard me and got snarky about it wondering what my true intent was for that comment. His sister came out and told him exactly what happened and was in defense of my daughter. He took the baby from his daughter and gave it back to mine and his daughter began to sob. The tip of the ice berg. She is starved for attention and has to be the star of the show for anyone who is around whether she knows them or not and will raise her voice until all eyes are on her. At her mothers house, she is the youngest and has two older brothers who treat her as a princess just as mother and all of her friends do. She was slow with her speech when I met her at 3 yrs old and I've seen videos of her hitting dad and it is laughed upon. As her speech was progressing and she was able to start forming words she would literally tell dad completely off and say things I would NEVER stand my children to say to me, but he would act as if he was not sure of what she saying and not say anything.

We ended up splitting up when I was 6 months pregnant with our son. I found a house in a residing town and all the kids thrived. Our relationship even thrived. We got along better at my home. I asked from the begining that he not bring his daughter to my home. He would take our daughter for a couple hours on the weekends he had her so that they could spend somet time together. 

Our relationship got to he point where we were doing really good, everyone was doing really good. We discussed getting back together and we talked about his daughter and the issues that were previously a big deal. We agreed that it would be best to try things out before we moved into anything too fast. He started bringing his daughter around and at first I had A LOT of resentment and found it very hard but powered through and was working on our relationship. 

He changed jobs which gave him more free time and time to spend with the kids. Our son was born and I switched jobs myself. The house I had rented in the meantime was too small so I was in the market for getting a larger home. We discussed it and decided that we were going to move in together. For the first 2 months things were going really good. The behaviors from his daughter were still there to a point but had gotten tolerable. Boyfriend doing very well at not picking sides and doing behaviors that were done before. 
We have now entered the down hill slide with vaseline. The behaviors returned with a vengence. We live in a highly populated area of other kids. When she is visiting on the weekends the other kids will not play with her and will make it a point to let her know she is not welcome. They will flat out tell her she is a brat or she is mean and they are not playing with her. You look around and see all these kids playing together including my own and then there is her all by herself. She has become violent with our daughter who does not speak but a few words but she will scream when she is hurt by her. I have caught her pulling her hair fighting with her over a toy. If I am outside I can hear my son yelling at her to stop repeatedly and when I go in to investigate he says that she is pushing him/hitting him/ect. She will not do the behavior in front of anyone. I have tried to be sneaky to see if I can catch her in the act and there is this twinge in her eyes and when you make youself present she will literally jolt her body to attention and get the 'oh crap' look on her face and quickly find something to do. 
My feelings have returned full force to the resentment, cannot stand, and no longer want her a part of me and mine as she is such a disruption. On weekends that she is there that my kids aren't for whatever reason I find myself occupying myself with work or other things so that I do not have to be around. I feel like I want to crawl out the door of my own home, from my own kids, just because she is there. 
This last weekend topped the cake. We were having my daughters birthday party so we were getting prepared for the family coming over. The previously stating situation with the hearing my son yell at her to stop occured. Me and her dad walked in the house and my son tripped her to get back at her for pushing and hitting him. My boyfriend lost it and freaked out on my son for being mean to her. I was so infuriated, I was shaking, I walked right up to his daughter who was crying but had a delighted look on her face watching this all occur and I did the exact same thing to her. 
Boyfriend stood there in shock, he then asked me if I was done with my behavior. It took all I had to not march up the stairs and wipe out her entire existance to my home. I refrained as I did not want to disrupt the day any further with the birthday party and told him I was done, I would expect the same out of him, and it sucks to watch such a behavior doesn't it.
I told him that we would talk about it later, knowing full well he wasn't going to like what I had to say about it, he fell asleep before we had a chance to speak so we talked this morning. 

I told him flat out that she was no longer welcome in my home. I was not asking him to chose and I fully understood if he left as well and I was okay with it. I can no longer risk the wellbeing physically but most importantly mentally over her. I regret thinking that it could work and we could be better. Had I known what I know now, I never would have gotten involved with him as she is the spawn of Satan. 

I have researched, had conversations, thought long and hard. I have tried to think of different ways to go about it, maybe setting a different schedule with my kids so there wasn't that interaction, but what about our kids, and what about myself. 

You would think one would feel horrible about telling your boyfriend that his kid is no longer welcome into your home but I honestly don't. If I never see that child again I would be alright with it.