We held a family gathering this time last year where BIL (DH's brother) met mine and DH's best female friend ("K".)
It was obvious to all except the unobservant that they were interested in each other and getting it on after the gathering. We didn't interfere or mention it. We spoke to both after in general terms and neither mentioned anything. Subsequent meetings and nights out with both they mentioned the other platonically.
We had another family gathering last summer where they arrived together, but not officially as a couple, and spent the whole time together.
K started telling DH and I about a relationship with a man who she didn't want to name. So she would talk in length about this "man" and how he lived a distance away and was in final stages of a divorce (all relevant to BIL.) we asked her outright several times if it was BIL but she flatly denied it.
We have spent a lot of time with K discussing this relationship without knowing who it is, and we are starting to feel like we are part of some sort of game but we're not sure why.
At Christmas they again arrived at a family gathering together (so we invited BIL but K came too.) nothing wrong with that, but puts us in an awkward position with K as we didn't know whether we should be extending the invite to K due to the lack of information we have.
We've just received a call from BIL saying he's coming to stay in our town but not staying with us because he feels it's too much for us with the DC (we've had him many times before so not too much!) And that K being the kind person she is, Is putting him up. And oh, he's actually already here and been here a few days already.
So obviously BIL didn't want us to get shirty if we hear he's in our home town and not told us (it's a small town.)
So - I assume they're together. Or they are pursuing a relationship. I know that it is none of our business, but they are technically the closest people to us. We have asked both outright separately if there is any romantic involvement and they say no, although that's obvious it's a blatant lie (as they get very touchy feely.)
It also seems like everybody else in our friends group knows about it. One told me the other day that she had been sworn to secrecy about something to do with BIL and she couldn't tell me 
I mean, this is all making us feel a bit like they think we are stupid? Or that they are playing some kind of game with us.
Do I sound like the noisiest person in the world? Or am I justified in my feelings?