Hands Tied need advise

After reading Delilah's blog (www.delilahmiller.blogspot.com) I had to post on my blog. I need advice or to just convince myself that I am not crazy. Here is my situation in a nutshell. I live with a guy who has 2 biological children and one other (11years old) one that he takes care of and calls his own. Only she doesn't live with us, she lives with her mother, mother's boyfriend (Jack) and babysister 3 years old. (not Ted's). This past weekend the mother Leslie, was at work and her brother who is 16 was at her house with his friend Chris who is 18. They were there with Jack, Kaylin and the baby. Jack is not a responsible person by all means, and he gave the two boys beer to drink while they were there, then after he gave them beer he went outside to talk to a neighbor leaving the baby and Kaylin in there with the boys. Now this made me very mad and I got very angry with Leslie and Jack. But seeing as how we have no authority over this child we have to be careful with our actions and our words if we want to continue to see her. I told Leslie that she is not to leave her alone with those boys anymore. Well just last night she did. Leslie works at a laundry mat 2nd shift and doesn't get off until like 11pm. Well Jack went to pick her up last night and left the two boys, the baby and Kaylin there. Now this weekend Kaylin was supposed to come over to visit because she hasn't been here in like 3 weeks. I finally get her to call from Chris's cell phone and she tells me she doesn't want to go to this little girls sleep over/birthday party because it is boring there and all they do is watch t.v, so I asked her well what do you do at your mom's house she said I play with the baby. I said well that can't be fun, she insisted that she did not want to go, so I asked her if she wanted to come over to my house. She said no that she wanted to stay at her mom's house. And that Chris and the 16 year old were staying the night. I come to the understanding that she wants to stay because of the older 18year old boy is there. She does want to come over tomorrow but just for the night then go back to her mom's house because she has friends coming over to stay the night so they can go to chuckie cheese with her on Sunday.
See here is another reason I don't like Mom, she planned a birthday party for Kaylin and T.J whose birthday falls on the same day but 5 years apart for this sunday coming up at Chuckie Cheese. Only the weird thing is she didn't tell T.J about the party or us so that way we didn't make plans that day or so she could make arrangements for him to be picked up/dropped off. And she didn't even ask him about it or see if he wanted friends to go, or if he himself wanted to go.
Someone please tell me that I am not insane and that this other person is crazy and a lunatic for treating her children this way and is there anything I can do about it????