Step kids and his ex partner

Please not rude comments... 
I seem to hate my partners kids, I don't treat them as if I do but I can't pick a thing I like about them and then there is his ex who seems to have as the time has gone on got into our lives more and more. They were together on and off for 10 years and so far we have only been dating for 1, I just feel like I'm in some constant competition until we have been together longer and I understand they still need to talk for the kids sake but chats constantly and him drinking with her and his family is not what call okay. I have said that to him but he just says it was his mums choose. What he couldn't say no? I'm probably being selfish. I don't know. This was more of a rant I guess, sorry for wasting anyone's time. I just don't know what to do. I've started being so stressed I can't even have sex with my partner because all I picture is his ex and she is a complete asshole of a person. Maybe I just can't handle the kids. /: I don't know. Any advice?