I’m not in that situation but I was the younger sibling I was 6 when my first nephew was born 8 for my second nephew 9 for my niece my favorite thing when they came to visit was getting to hold them and feed them and play with them it was something I always looked forward to. Now for the first couple I always had to sit down before I could hold them and I couldn’t just take them I always had to ask but for my nieces I didn’t have those rules I got to hold them because I proved that I could handle it. For me I understand your rules especially since they aren’t following them but maybe giving them things to work up to to show they can handle helping and following your rules because he is your son
Personally I just think they’re trying to bond with their nephew although an 11 year old is little not all are immature I have a 12 year old sister she takes care of my baby ( she’s 4m) she even feeds her and burps her because she wanted to bond with her neice. Don’t see it as a bad thing it is frustrating for kids to not listen however try to come terms with it if she’s not causing any harm I don’t see the problem