if you don't want to tell your aunt, dont!
If you do want to tell people, but want to stop anyting negative before she starts with it you can try telling them something along the lines of what I told my mom (3 days before surgery lol).
Basically I said "I am having lapband surgery and while I appreciate everyones concerns and thoughts, if they aren't going to be supportive of MY decision, then I don't want to hear it. It was my decision to make, I made it on my own, and it wasn't made lightly and it isn't the easy way out as some people think. I don't mind questions because I know you don't know a lot about it, but please don't try to talk me out of it, it wont work and like I said, it was MY decision. I need positive and support and if you can't do that, that's fine, but I don't want to hear it"
It went surprisingly well. I had to be firm in the way I told her and of course there were a million questions but I'm 2 weeks post op and she's been nothing but supportive (my sister too...and her initial reaction was "you're crazy").
Good luck!!