We were the same with joint a/cs and always very transparent together about money.
Unfortunately last year, my H started removing thousands from our joint a/c "because he needed it".... please focus on you and S13, it's not being spiteful, just practical and you can give her the opportunity to share in the discussion. But have a plan for if she doesn't.
And if she is seeing someone else, even if he's paying for swanky dinners etc, your W will be spending money on new clothes, hairdos etc....
Me: 53 H:38 T:20 M:14 BD ILYB etc 10/15, H diagnosed severe depression S 1/16 PA 4/16 H filed 1/17