I think that there is something to be said for accepting that some people just cannot be in your life. There sounds like a whole backlog of hurts which have been stifled but as you say, if the relationship with your SS and his partner have always been strained, rejoice that you have accepted it is over and you no longer have to walk on eggshells. Why should you have to? If they resent you, let them get on with it.
I only say this because recently a rift was created in our blended family. I hate confrontation, like a peaceful life and do tend to bottle up my real feelings for the sake of harmony. I was grateful to a friend who after listening to my tale of concern, woe and all my hand-wringing and angst said quite bluntly. "Look, no more fake smiles for you, no more feeling uncomfortable, or having to try too hard to get her onside, and no more going along with things you find unacceptable. It is much healthier for you to accept you'll never see her again. Get on with your life, with one very difficult relationship gone"
It is difficult when it involves family members and split loyalties. It's hard, but as some wise person said, you can choose your friends, but not your family. It takes two to tango, two to argue, two to fall out. Sometimes it's futile forever trying to be the peacemaker and the one making allowances and amends, as I was.
Enjoy your day and wish your step family well but let it go and move on.