I Need Help

ok i have really go to talk to someone i have a situation that i cant seem to figure out..ok so my dh and i are expecting a baby in february and we are living with his mother right now which is some what ok for now.. but his sisters kids are here to and we are practically rasing them their mom and dad dont have anything to do with them.. and it really bothers me because for one i hae to sit and hear this little 4 year old cry her self to sleep every night wanting her mother.. wll thats not the main problem though.. see she does not seem to care how her kids are or anything she doesn't call to check on them she doesnt come to see them or anything. and see well she was so called dating this guy well thats what she called it but in all reality she was really just using him for his money.. he broke up with her cause the house that she is living in he was paying for and she had other men in and out of there while he was out of town working (he is a truck driver) he loves these kids to death and he would do anything in the world for them or her and she jusat used him.. well now she is saying that me and my husband are plotting agenst her just because we want her to spend time with her kids.. she is trying to break me and my husband up and you know its hard cause i cant say anything to her cause i dont want to stress out more than i already am i mean hell i get at the most of 4 hours of sleep and i have around 15 weeks left till by due date and i just cant deal with more stress. what should i do to be a better parent for my kids and a better role model for her kids with out dealing with all the critasisame?