Well, as Minixxa said - people are going to talk - regardless !
Know what I'd do ?
Direct their conversations. Be in charge of what their conversations are. In other words - give them something substantial to talk about !
You could have swam between some legs too and been horsing around. And in some way that was obvious but NOT tasteless, making it public that you and her are very much on the same page and have your own bond - be it 'sisterly' or otherwise and that whatever interactions are occurring with him and her are FULLY sanctioned by you !
Then sit back and watch the fun