I was in a similar situation, I was 14 or 15 and I was looking after my 2 cousins who were 10 and 7 at the time. Between my aunt/uncles schedule, I was at their home almost every day and on weekends spent a lot of time with the kids, and felt obligated to act as a fill-in parent. Even at 14, was helping them complete their homework, ignoring mine of course, making them meals and disciplining accordingly. The time spent with them was reassured when I would be over with my mother and sister, and the kids would not listen to them, but acted well behaved/respectful to me. I worried if my relationship with my cousins/aunt and uncle would change but I moved out West and don't speak to my family as often as I would like to, Purely instinctual and as long as you're self aware that this could be a problem, I wouldn't worry about yourself. If anyone has a problem, remind them its about the child and not who appears to be the parent. Toodles!

Can't say that it's a bad thing at all! all that you listed are in fact, very good things. Its good your teaching your sis all of those things. I can't really relate though, because my brother and I are 6 years apart and he wouldn't listen to me at all if I tried to teach him anything. He listens a lot more to our parents than to me. XD haha

There's nothing wrong with you for having that sort of connection. I feel this sort of feeling in regards to my little brother because he's autistic and needs the extra guidance.