AITA for telling my sister she is required to wear a bra at my wedding

I love my sister. And truthfully when I say required I'm obviously not going to kick her out if she doesn't wear one but like she really really really needs to.

My sister is well endowed and hates bras. When she's wearing certain items like baggy t-shirts it's not so noticeable but in the dress (that she chose, I just gave her a color) she's wearing in my wedding it will very very obvious she's not wearing a bra and one wrong move and she could be flashing everyone. Not only does it look bad, but it's also borderline inappropriate.

I told her let's go shopping and she can try a bunch of different ones, I even found a boutique in our area because stores carry very few options in her size and said I'd pay for whatever she picked out.

Now she's saying I'm playing into sexist stereotypes. I'm saying that there's a good chance if you dance too hard in this dress your titty is going to pop out and I don't think thats sexist.

AITA