I would like to file for full custody of my stepsister but my mom...?

You need to but the pros and cons of this situation. Your mom is a grown women unhappy of her life that's why she does what she does, Like I said she is a grown women capable of taking care of herself on the other side Sarah has had a super hard life she needs you support 100% of the way, you need to protect her from everything and everyone including your mom. Sarah only has you!! Do whats best for sarah. Don't worry of the consequences do what you have to do. This world need more people with your kind of heart to help others. Sorry but child support should go to Sarah and where she lives.