What should be done here?

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I have a stepdaughter who is eight years old and she lives with her dad and I. At first her mother wanted to spend time with her all the time because she was jealous of me being around her daughter. I suspected something early on because there was one day when he was supposed to have picked his daughter up at 8:30 in the morning and it was 9:00 when we woke up and she was banging on the door and when he opened it she said "I told you to come and get her at 8:30 it is 9:15! Don't bring her a$$ over there no d*** more until I tell you to! She said all this right in front of the child who at the time was four. Soon after that he would like call her and literally beg her to watch her own daughter. So one day he was talking to me about her and why she was acting like that. I said you shouldn't be calling her begging her to watch her own daughter if she wants to see her all that bad then she would call you sometimes and ask you to bring her over. So he stopped begging her to watch her own daughter and she didn't call her or come by to see her for two whole years and the sad thing about it was that we lived five minutes away from each other. I have came to the conclusion that she just doesn't care. To me that is what I get out of this I mean she lives five minutes away if that long and she won't call her, come by and see her, and she doesn't give him one penny to help take care of her. Tell me what u think? The other day my stepdaughter was crying and I said what is wrong with you and she said Why my mom always want to be with my sister and my brother, but not with me? I didn't want to tell her how I really felt so I just tried to make her feel better about it. Mind you that her other two kids don't live with her either, but she is always seeing them. Do you think that she cares about her? What could be the explanation as to why she does this? I had talked to her one day about it and she just pushed all the blame on to my husband saying that he was keeping her away from her. She tells everybody that like it is some kind of joke or something. What should I do to help the child understand that it is not her fault?