I have two kids of my own, 12 and 13 
I also have 2 stepkids, the boy is 10 and his sister is 7.
DH and I have been together for 4 years.

I have a few things to unpack here and would be grateful for some advice, 

Stepson is a difficult character. 
In that time I’ve seen a lot of bullying behaviour from the brother to his little sister and I’ve told my partner about it, he will have a chat with the kids but it’s never really ever been dealt with, the kids mum is oblivious and insists they are very close siblings and they “never fall out.”

I’ve seen the brother pin his sister down and fill her mouth with grass when she was saying no, I’ve seen him kick her down on the floor and then put his foot on her chest to keep her down, he used to pull down her knickers (thankfully doesn’t do that anymore) he swears at her, shouts at her and insults her. She cries constantly. He’s also got behavioural issues, he’s addicted to his PlayStation, gets aggressive when he’s told he can’t play it and is addicted to other devices, he will get up at 6am and be on the device all day if he could. He lies about things all the time, especially about what he’s doing to his sister, lies about his bad behaviour. I’ve never heard a kid swear as much as he does. He’s ok around my two but he takes everything out on his sister who is becoming withdrawn, crying all the time, and now she is acting out, wanting attention from other boys so she develops crushes on boys in her class. She’s desperate for attention from her brother. And is becoming manipulative, difficult and whiny herself but I know it’s because of his behaviour and she’s fine when she is on her own with my two. 

Not sure what to do anymore as both parents don’t seem to want to accept his behaviour is awful because they are invested in the idea of their kids being best friends. DH is very gentle and kind, and the kids mum is more concerned about looking perfect on the outside and pretending everything is perfect, so they aren’t protecting their daughter from this bully. Their mum is very wealthy too so her kids get everything they ask for and he’s very entitled which means he also expects everything to be given to him that he asks for and has been complaining to her that he doesn’t get to do fun stuff at our house which is why he picks on his sister. 

My kids aren’t angels of course, but the only time there is peace is if the stepkids come over separately because he wouldn’t try to bully my two because they are older and they keep to themselves. When we have been out with other parents and kids he will cause scenes and accuse the other younger kids of doing things to him and pretend to be injured for attention. 

I have rules in my house about his behaviour and recently have called him out more on lying. He will cry and say he misses his dad and use it as an excuse to get out of admitting to doing bad things, neither parent will follow through on holding him accountable. 

I feel so sad for his sister and I’ve encouraged my two to separate her from him, but it’s not my job to do what neither of his parents seem able to do. 

It doesn’t feel right to do nothing, but I’m not sure what else to do. I’m starting to find him to be quite stressful to be around because of his bad attitude. Things came to a bit of a head this weekend and I’d like to have a bit of a coping strategy in place for the weekend when we next see him. Just to add we have the kids every weekend. I think what has upset me is that he he lied to his mother and said he never gets to do anything fun here when we do stuff with them all the time and we don’t have a lot of money right now. He’s using it as an excuse to be mean to his sister.

After everything came to a head this weekend, I told DH I’m starting to not like being around his kids, I said that I think the reason he is so lenient with them is because of misplaced guilt and he agreed. 
We’ve chatted about it this week and he’s agreed to put some boundaries in place. He’s probably going to take a bit of time to come to terms with how he has been previously as DH has Aspergers so I have to lead him a little, but I’d like to have a bit of a plan. DH is very tolerant of my daughter who also has Aspergers so I don’t want to be a bitch! 

At the same time I don’t look forward to seeing his kids which breaks my heart as I love kids and have looked after hundreds of them.