Girl, you are just going to have to talk it out and see what happens. It's your aunt so you love her, she loves you so as uncomfortable as it may be I think you cannot avoid the subject. If it turns out that she gets super offended or something, well at least you tried to work things out. But no, I would not be cool with anyone doing that to my child and I would bring it up and ask some questions. You could start with something like "Hey did I understand correctly that you put my daughters thumb on her private area as a correction for thumb sucking?" She may then offer details like that she just swiped it in that direction to show that thumb sucking was nasty, she may say she put it there under the clothing, who knows. But you need more info for sure. Then just tell her how you feel about it. Try to go into it with an open mind that you might be missing some info, but if what you find out is just totally disturbing at the end of the day your kids emotional health comes before even their desire to see their aunt, hopefully things will be cleared up and worked out though. Best of luck, that one would have thrown me too.