My youngest sister can't talk and has poor motor skills and basically the intelligence of a really well trained dog.
Growing up with her sucked, to be honest. She was at her worse when I was in middle school, so I was at a bad age for that. She would hit and bite, and do this weird masterbation thing in public. For a middle schooler it was horribly humiliating. She would have violent outbursts all the time and my mom would just start crying. We got kicked out of our church because they wouldn't take her in the daycare and we couldn't have her in the service (which was a nice perk for me because I hated going to church, but it was still humiliating). She used to wander out into the street and do her dance. It was not a safe street to be on for her as cars sometimes drove way too fast, usually driven by teenagers and drunk drivers.
She started settling down as she got older and now is fairly self efficient, in the most basic ways. We still can't really leave her home by herself for more than 15 minutes or so, but she can get her own snacks and go potty by herself. She still needs help bathing, which is awkward when I have to do it because I'm watching her for a few days. She goes to school and absolutely loves it, it's like really really special ed, but she understands some sign language and she understands verbal commands. She LOVES In N Out and meeting new people and seeing people she knows. Everyone loves her because she is really sweet now. She still can't talk, but she certainly is vocal. I moved home after grad school and ended up taking care of her a lot. Oh, and to this day my younger brother couldn't read the newspaper because she will just start hitting him for some really weird reason.
She is my favorite sibling, despite all of the hardships of growing up with her. I don't know how it really benefited me other than having punched a couple people in the face for making fun of her. We surely make fun of her in a family way, and I allow certain close friends to do the same. It seems that people are becoming a lot more understanding about these types of situations situation these days, so that's good. I really wouldn't know anything else because it's just how I grew up. She's 22 now, just so people know.