Is it strange for my mom to hang with my ex?

My mom and my ex have become close friends since he and I broke up. We were together for 7 years, during that time he cheated numerous times including with my best friend. We have 2 children together (I have 1 from a previuos relationship and he has 3) and he barely works so I have always had to support myself and my 3 children. We broke up last April because I was tired of supporting him and being emotionally abused. My mother has since taken him under her wing and this is driving me crazy. We live with her so he is around everyday (he used to live here too until there was an issue of some missing oxy pills); sometimes doing things with the kids (teaching them to ride bikes or taking them to 6 flags) but a lot of the time he spends hanging with her. My younger sister also lives with us and even she has become very uncomfortable with their relationship because she knows how it makes me feel especially since he still does not contribute financially. My mother charges me rent and a fee for my two oldest in the summer that he has made it clear he will not help pay. I intend to move out within the next month, and have been considering cutting ties with them both when I do, but I know that's the wrong thing to do and I only feel that way because I am angry. Recently, he and I got into an argument and he got in my face and asked if I wanted to fight him. I know this was only an attempt to scare me and I have become immune to such from him, but I told my mom and she didn't even react. They hung out together at my aunts house all night and didn't return until the morning. I know the only thing that will make me feel better is to get myself and my kids away from this confusion but am I wrong for feeling like they are wrong for having this friendship when it so clearly upsets me?
Is it strange for my mom to hang with my ex?
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