Not everyone goes through a baby phase (or becomes a baby) in their adult life. What is important is that you find out how old you are. One way is to look at your interests. Usually people growing up tend to watch stuff with characters just a few years older than they are. I think this has to do with a fixation on growing up since well it is that time in their life. If after a trip into self discovery you find that your mental age and your physical are the same, the honestly the best thing you could do would be to open up to your sister if she could/can/would keep your secret. Then you would be living with someone who could help teach you and she would probably be more than willing to give you some of her old clothes as long as you don't ask her for intamate things. She might even find it thrilling to have a new secret little sister". Be really nice to your older sis and you'll not only become better friends with her, but gain an ally if you ever "come out" as something.