Do you think what I did was immature?

No it was not mean.. ur stepmom sounds like a major b****. The only is I wish you could have stood up to her sooner. She needs to be beat down... If your sister is unhappy I would definitely find out what you can do to get her out the house. I would call child protective services on ur stepmom.. as soon as they see what's wrong they would not hesitate to keep her safe... Have your sister talk to a teacher or counselor a school and if they feel that any type of abuse i going on they will definitley contact the authorities. and Im sorry to say but ur dad is a.jerk I would tell him off too..its sad.. I hope and pray everything works out for you and your sister