I'm going to take a wild guess and say it's because he wants the attention and adoration being baptised will bring ( for about 5 minutes). As a boy he might also enjoy wielding power over his big sister - DA DA DA DA....DUBMAN !!! You could make him a cape LoL!
Srsly tho your parents don't seem too bad if they are happy for u to just pretend, and they won't force you. I would only recommend a move in with Aunt if u know her very, very well and she is totaly willing to take on all parental responsibility. You are still young. Go easy, work hard at school and aim for college.