Sister is a recovering drug addict

My sister has been clean for a few years now and I’m really proud of her. She’s still not all there, but she’s not on drugs and that’s the most important thing. Whenever we talk about our mom she always says MY mom. I find it really strange, but obviously don’t want to get on her about something so small. I just don’t understand why she always says that. I just say Mom. She’s our mom. Never have I said my mom when talking to my sister. I don’t get where her logic is coming from but she doesn’t really have the best common sense so I should probably let it go?