Prepubescent SIL

I have six months old twins. My parents live half an hour away and my In-Laws live fifteen minutes away. I have an eight year old SIL who is obnoxious, rude, and an all-around-brat. She feels like she can yell at my babies when they spit their binky’s out or when they are crying too loud for her to hear her computer game. And no one says a thing! Her parents will sit right there and let her do it, and I don’t feel like I can yell at her because she’s not my child and I wouldn’t be respecting them if I did.

I let my boys stay over at my parents’ house often and stay nights sometimes too. They are very comfortable with them and I have two older sisters that still live at home to help them out if they need anything. My husband’s parents ALWAYS want to watch them and I don’t want them to since it causes problems between my husband and I when I tell him that I just don’t trust them with the babies or that they aren’t comfortable enough with them yet, or heaven forbid I say anything about is “angel sweet” sister, and how if his parents can’t watch their own eight year old, how am i supposed to trust them with my two six month olds??!!!!