Family does some strange strange things.

Saturday I went to the annual Christmas party the honey's family holds. While there things got a little strange in the middle of all the fun I was having.
So the honey's brother and his long term girlfriend Diane (almost as long as me and the honey) show up at the Christmas party from Nevada with her daughter and her husband. I am talking to Diane after the hugging and helloing and she holds out her hand and says look at what Tom bought me. There is a sparkly on her hand. An antique type ring. I said wow that is pretty. She goes on to tell me that they were in a antique store and he bought it for her and asked her to marry him.
Now I went into shock because Tom is a hard core bachelor. He was burned hard when he was young and married and it led to a divorce. Ever since then he has been a never in this lifetime am I going to be married type of guy. (he is now in his 60's) So it was a big surprise. I hug her and she goes off to show the rest of the family the ring and tell them they are engaged. I was seriously blown away. So in fact was everyone in the entire family.
About 2 glasses of wine later (for me) I run into her daughter who must be on her 10th beer lol. She grabs me by the arm and drags me into the other room and says you know that whole story is not real right? I look at her and say what story. She says the engagement story. I pretty much say WTF are you talking about. She tells me that Diane is worried that because she has never seen Toms will that the house that they paid for and all the other stuff will go to someone else besides her. Even though Tom has told her repeatedly that she is his sole beneficiary. So in order to firm up in her mind that the family won't try to step in and take what is hers she came up with this idea of an engagement.
Now I am really sitting there with my mind blown. I am thinking WTF. How did she even get Tom to go along with this? Did Tom even know she was going to do this? How could she not be honest with me when I am pretty much the only one in the family who really is close to her? I mean hell I have spent many days and nights with her.
The daughter then makes me promise not to say anything to anyone. Then Diane walks over after my next 1/2 glass of wine and says Melissa told me she told you. I just looked at her because I didn't know what to say. She then proceeds to tell me not to tell anyone anything at all at which point we are interrupted by a group of people and I hadn't said one word to her about it. Honestly I had no idea what to say. I mean that is a big ummm ruse to pull over on an entire family at a Christmas party.
A couple of hours go by and I am sitting on the floor talking to the honey's nephew's girlfriend Bailey. I see Diane and Melissa on the other side of the hallway and hear Diane say to Melissa that she had asked Tom what he thought about what was going on and wiggled her ring at him and he said well maybe we will have to make it happen. At which point Melissa gets all freaking giggly and happy and starts hugging Diane and telling her this is the greatest news ever and she will help pay for it. Then they walked off.
So here I am now in a very strange position. Everyone including the honey thinks Tom and Diane are engaged. Everyone but me who knows the truth. (although I am curious if Tom knew she was going to do that because let me tell you he is not the type to do that.)
So here it is days later and I am still confused. I mean really I could maybe understand doing that to your friends but your family?