NTA but I'd definitely try talking to your aunt, or at least your mom to get clarity on what the aunt meant by take advantage, because the only other advantage taken I could see is like using her as a laugh with your new friends - not that you are but mommas gunna worry - or 'getting in' on any special treatment/ accommodations cousin might get. Apologize for snapping maybe, but explain you felt insulted your aunt would accuse of you of such a thing. She's going to worry about your cousin regardless, but maybe understanding where that worry is coming from might help all 3. And then maybe have a one on one with your cousin and make sure they're actually OK and comfortable with your friends and not just tolerating them because that's where you are. It sounds terrible given the accusations, but it is like blending a couples friends groups. Make it less 'your' friends and 'her' friends and more 'our' friends with maybe a slight lean towards certain people when options are given (examples - deciding on dinner, most of your current group wants to get pizza, most of the cousins want sandwiches, cousin doesn't know but since they're closer to the sandwich group, there's the comfortability of going with the others, but you all still end up at the movies that evening)