All the people saying “OP should spend that time with her SIL instead” what makes you think the SIL who has been cheated on, and then unceremoniously dropped by people who WERE her friends and family would want to see OP?
If she’s childfree for the day, she’s probably already made plans and they won’t include awful people who drop her during one of the hardest times of her life 🙄
I’ve been in the OP’s situation (albeit without a controlling arse for a DH) but I stayed in contact with my SIL throughout her shitshow of a divorce and my BIL trying to hide money and swindle her out of the family home. We built a solid relationship with my niece and nephew, who she had main custody of and whose world had been turned upside down.
After a few years and the divorce settled, my DH met his DB and the OW. DH then asked me if we could meet them as a family as it seemed like she was here to stay. I dithered about this decision for a while, but decided that I would for the sake of my DH’s relationship with his brother.
I have to say I have resting bitch face and my disdain for the two of them was evident throughout. The OW had been a “friend” of my SIL and knew the DC etc, yet had completely fucked them over because my BIL had money. I made it through the relatively short lunch and told my DH I wouldn’t be seeing them again, they’re just not my type of people.
I needn’t have worried; two months later my BIL had emigrated with a second OW, leaving both his DC and the original OW in the UK. The OW found herself partnerless and homeless as my BIL sold the house she was living in and I’ve never seen her again.
My DN & DN are now adults and we have a wonderful relationship, along with my SIL who is a delight to be around. I haven’t had contact with my BIL in years, or ever met the 2nd OW and I haven’t no desire to.
In short, I’m an advocate for spending your precious time with decent people and dropping arseholes (blood relatives or not) because decent people will enhance your life and arseholes will always be arseholes 🤷♀️