You may not be able control your sister’s choices but you CAN be a safe space for her kids. Nothing is stopping you from being the cool/dorky aunt. With my nieces, I never asked them to “report” on their mom. But I kept track of their school events and their interests so I could have a conversation with them. We went on occasional day trips to the zoo and whatnot. Comments about my sister’s drug addict boyfriend were few and far between, but when they happened I made sure to hold myself back and not tear them apart for details. (Although I kept on the lookout for any signs of danger.) You may not be able to save your sister from her decisions, but you can be a resource for her children. The trick is to make very clear that you are not “competing” for their affection but that you just love them as they are.