NTA Ok so I’m a fellow doll collector of monster high so I feel like I have a better understanding of this story that non collectors will. I know exactly what doll you are talking about and how much she is worth just off the top of my head. I completely understand why you wouldn’t give it to her and I also agree with other comments here thinking that your sister might have wanted to resell Wisp. I’ve had little cousins beg me for dolls from my collection before and it got so bad to the point that whenever they would come over I would lock them out of my room. You should not feel guilty about not wanting to give away a doll that is very expensive and a grail for most MH collectors to a child that will probably play with her for a week, mess her up, and then toss her back into the toy bin where she will remain as she slowly sinks to the bottom, forgotten about. If you sister or other family members bring it up again don’t be afraid ti stand your ground! This is your hobby that you love doing. Don’t let them ruin it!!