So I had this happen once.
My sister met my best friend (at the time) on my birthday. They ended up getting really drunk and hooking up on my living room floor.
I was annoyed.
My sister said he gave her butterflies.
My friend said don't worry about it.
I told my friend that the scenario didn't make me happy and that I didn't want to hear about it. So my friend and I are both in Minnesota/Wisconsin at the same time. He lived in Somerset, WII I lived in Stillwater, MN. Anyway I'm over at his house and I ask who is picking him up now (I was supposed to, but I ended up going to MN). He says that my sister is. Not a moment sooner he gets a text from my sister. I will not state the contents of the text, only that it was something you'd expect from a pornstar... not your sister. He laughs and says hes sorry and that hes just enjoying the sex and that she is a bit too strung out for him to really be interested.
I get home and I'm talking to my mom and my mom is going on and on about how my sister thinks that my best friend is the one and blah blah blah. So I tell my mom that I don't think she should get her hopes up and that he is a bit of a man whore.
I let it die.
A few weeks later Im on the phone with my mom... We are arguing about something stupid and shes all."well from what I hear, you aren't exactly an angel or anything." I ask what she is talking about and apparently my former best friend let my sister know all my exploits in which my sister was angry when my mom told her to not get her hopes up so my sister told my mom everything that my friend had told her.
The aftermath.
My mom thinks I'm a whore. My sister and I took about 3 years to repair our relationship and my former bestfriend stayed friends for a few more months until he yelled at me for not wanting to go out on the night that my grandfather died.
SOOOOOO..... I'd say its a horrible idea.