I genuinely don't care that my wife's grandmother is dying
Why don't you care about your wife's dying grandmother?
She was one of the worst people to ever exist. She beat her granddaughter all the time.
It's understandable that you would harbor resentment towards here. You should probably still show sympathy, though. People you do care about will think better of you for it.
Maybe you have a point.
They say funerals are not for the deceased -- they're for the living. If you keep that in mind, you'll find it easier to attend.