Awkward Thanksgiving

My SO and I are going to his mothers for dinner this year. It’s usually his 3 siblings, their SOs, his aunts and uncles and his grandmother. This year his older brother is bringing his new fiancé. I say new because they’ve been together less than a year and I have never met her, but i already know a lot about her. I know she moved here from out of state to be with him, i know she practically supports him financially, and i know he cheated on her... multiple times... with different women and she caught him. I also know that NOBODY is supposed to know this. His Dad is the only one who is supposed to know, because his father essentially talked this woman into taking his son back. Well Father let slip to my SO while he was venting about his brother’s idiocy. Then of course he told me... and then his mother. Whoops! Last time their was a big secret let slip at one of these functions, the family didn’t talk for months. I’m not looking forward to this.