I've heard of instances, but I don't know if I'd be ok with it. I would just be worried that if things don't work out and end badly, things would be quite uncomfortable. In the end, though, it's up to them.
I think it's great as long as the relationship works out. I'd be concerned about what it would be like for them and the families if things don't work out, but why "borrow trouble?" If that happens, it can be dealt with then.
I don't see anything wrong with it personally, but it could make things a bit awkward for some family members. Not everyone is as open-minded as I am, so I'm not too sure how the families would handle the situation.
I think that it would be a bit weird. Families generally don't get on that well once the extended family starts to run into the closer family, and that seems like a bit of an overlap that I couldn't handle.
I agree with the guy who asked about them splitting. What kind of waves is that going to cause within the family? I really think that it would be somewhat awkward.
Awesome.... all in the family. Are you close with your sister in law? That would be kind of cool to have them date and fall in love and get married. I don't know anyone who has done this.