I'd like to learn ASL sign language + teach my little sister (5). Do I start with baby sign? Adult?

I think if your little sister can't sign at all yet then maybe start with baby sign and then move on to other signs like animals, colours, food items etc. Make it "easy" and fun for her. You can teach her the alphabet letters/numbers too at 5 years old. Have a look on the ASL kids website, you'll find some good information there. Good luck and good on you!! :-)