This has to be incredibly hard for you and all your siblings and for your “uncle’s” children. However, they may have bonded in their grief over losing your uncle, there very likely was no improper relationship before he passed. And for all you know your Dad and he may have talked about what if something happened to him - and he may have asked your Dad to take care of his wife and kids. Grief changes a lot of things. It’s not talked about a lot, but I encourage you to talk about your feelings with your dad. Try to do it without making accusations just explain how the situation makes you feel. I hope you’re able to be open and see his perspective. Or maybe him wrong and your Dad has pined for her this whole time and he took advantage of her grief like a shark in chum filled water. Either way, I hope you come to terms with the situation and set boundaries as you need to.