SUBREDDIT: r/relationships
TITLE: My sister[12/F] and I[17/M] have found out our Dad[56/M] is cheating on our step-mum[53/F] and we don't know what to do.
POST: So my sister and I have found out our dad has been cheating on our step-mum for a while. I want to tell her but I'm not sure it's the right decision since she has given up everything to try and accept us into her life after we moved in with her around 5 years ago. She has supported him through his £20k+ debt and has moved multiple times so we can all live together.

If I were to tell her and they split up, do my sister and I get a decision in who we get to move in with? Not sure how it really works since they aren't married.
TL;DR: 
Dad is cheating on step-mum and I don't know if it's better to stay quiet or to tell her after everything she has sacrificed for him.