Your Dad is the one who needs most support now. Bite your tongue.
Be the oil on the troubled waters - by maintaining your own calm and being civil with everyone. Not easy.
Meanwhile come and character assassinate the bitch anonymously on the internet.
I have a feud already ongoing with my brother, haven't seen him for 12 months, don't particularly want to see him ever again... but will be interesting when the old chap shuffles off the mortal coil which could be any time in the next few years.