My fiance's family is going through this right now. There are 3 children, all have handled it differently so I'll tell you which actions yielded the best result.
His one sister lives very close (almost next door) to his parents and she's involved in EVERYTHING. If their dad wants her to "stalk" their mother on FB she'll show him how. It's really affected this sister, more negatively than I think she thinks it has. He relationship has kind of fallen apart as a result of all the tension.
My FI has taken a side. The "rightful" side, IMO, but he's been a constant ear for his dad, however, his dad doesn't LISTEN to any advice given and it's an overall depressing atmosphere.
His other sister has completely ghosted. She went and lived her own life and leaves them to deal with their issues. We haven't heard a peep from her in over a month.

My advice? Be the other sister. It's not of your business and they need space to work out their issues.