why does she hate me

My partners ex wife HATES me. She can not stand me at all. I haven't done anything to her, or their children. In fact, I look after their girls like they are my own. I feed them, I help them with homework, I clothe them, bath them I even discipline them when absolutely necessary. I didn't break up their marriage. We have been together for 4 years and they have been divorced for 7. I just don't understand what I did to make her hate me so much. I try to be nice and civil, but she gives me the dirtiest looks and ignores me. Their youngest daughter is 5 and she told me that her mom was angry at me and wanted to punch me and the 9 year old said shhhh she didn't say it like that. She refuses to be in a room with me any longer than she has to. I've been nothing but nice to her I just don't understand. I'm a really nice person to everyone so I don't quite find this fair and it is very frustrating