Going no contact is never a jump up out of bed, this is what I’m going to do today decision.
It is a final resort surrounded by years worth of pain and trauma, regardless of the instigator. The only individuals who will really understand are the directly affected ones, in this instance SIL and her father.
The situation with no contact often appears very black and white to outsiders - he was abusive, she was spoilt, he acted like a dick when xyz. It’s rarely so cut and dry. As the adage goes - there’s three sides to all stories - this side, that side and the truth.
SIL has felt so pained that it’s a necessity to cut contact with half of her DNA. That’s a very traumatic decision to make with lifelong impacts. Her father died without her saying goodbye. Whilst, yes, that was the intent whilst being no contact it’s still a terrible burden she will have to live with and you can guarantee not the outcome she ever wanted or envisioned for her life.
She may want to go to the funeral to solidify the fact he is dead, she may want to go for closure and to say a silent fuck you, she may want to go to apologise that they never had an opportunity to reconcile, she may want to grieve for the man he used to be. You don’t know. What you do know is that it is none of your business or concern whether she decides to attend.