Bride's Maid Selection

I have an unusual question....I have already selected my BM, but I recently discovered she had previous relations with my FI, a long time ago, I need advice, should I leave it in the past? I am breaking out in hives thinking of this. Please help!

Tabs

if you are good friend with her, leave the past in the past. if you aren't that close, then reconsider.
how did you find out? how long ago was it?