Is there something wrong with my sister, or am I just being overly critical?

I'm not a parent, but a sister. I'm 16, and I'm very concerned about my sister, who is a couple months away from being 13, and going into the seventh grade (our middle schools start in 6th, so she's already been in middle school for a year). I may be overreacting, because I've always been fiercely independent, but my sister is going on 13 and can't tie her shoes. She is physically able to, I've seen her tie an apron when cooking, tie a bow on a present, etc., but my mom put on her shoes and socks for her every morning until she was 11, and then my mom put her foot down and refused to do it. After that, my sister refused to wear socks and pulled her sneakers on and off without untying them, and she still does this. She has never brushed or styled her own hair in her life, my mom brushes and styles it for her every morning, usually in a plain ponytail or two braids, nothing complicated, and she knows how to braid hair as well, but she won't do it. She tends to get frustrated and throw a tantrum if she has to try at something (i.e. homework) for any more than half an hour. She also makes sure one of our parents stands at the end of the driveway and watches her walk to the school bus stop, which is only two houses up the street, there are usually about 10-15 other kids there, and we live in a very safe D.C. suburb. She also sleeps with every light in her bedroom turned on. When she was 7, our cousin told her, obviously jokingly, that there was a hobo living in the trunk of our mom's car. She still checks inside the trunk every single time before she gets in the car, and refuses to sit in the third row of seats in the back (it's a minivan). I think she also may have some serious body image issues. She should probably be wearing size medium shirts, but she refuses to wear anything but size XXL. When we go swimming, at the neighborhood pool or even at our lake house, which has a completely private and secluded beach so only other family members are at the beach, she wears a girl's one piece swimsuit, and then puts on boy's swim trunks and a rash guard, essentially going in fully dressed. She used to have a huge group of girls she was really close with, and this year she only wanted to invite two girls to her birthday party. I think her old friends have distanced themselves from her because, well, they all act like 13-year-olds and she acts like she's 7. I tried to bring all of this up with my parents (obviously in private)about a year ago, and they told me to "drop it and stop putting your sister down". Are these serious issues or am I just being overly critical and mean? As I said, maybe I'm overreacting. But my parents are acting as if she's a normal middle schooler and absolutely nothing is wrong. I feel that her behavior really isn't normal or healthy, and I'm really worried about her. If these are serious issues, how do I bring it up with my parents? If they're just going to reject what I'm saying because I'm her sister or because I'm 16, could I do something like talk to an extended family member or our pediatrician, and have them carry the message on? Would my parents listen to them? Oh, and if it makes a difference to the situation, I also have a brother who is 9 and has none of these issues.