Hey honey.
Wow, it sounds like you have a lot of anger. Understandably, it sounds like a pretty rough situation, with a lot of people handling things very.. very badly. It sounds like your sisters' mother is incredibly immature, and it's unfortunate that your uncle felt the need to "take someone's side," in the matter. He's screwed himself, because he's cut out a brother and a friend, and kept himself from getting to know you and the rest of your wonderful family, including your mother... whom he had no right to punish by snubbing her for your sisters' mam's issues with your dad.
Take it or leave it:
I have a hard time forgiving people, I can hold a grudge like there's no tomorrow. It's a talent, I think.
That being said, there's something about forgiveness... when I hold grudges, they eat away at me, I get emotional, angry, frustrated.. it's such a weight on my shoulders... and when I make things right with someone, it's this... fantastic sense of freedom from all the horrible tension. I love that feeling.
My mom's dad cut and ran. When she was about 15 years old, he left his wife (my grandmother) with 5 kids to raise on her own, and disappeared off the face of the planet. No one saw or heard from him for... jesus, my mom was 15, and she's 45 now... so no one heard from him for 27 years. They didn't even know if he was alive.
Then, 27 years later, they got an e-mail from his new girlfriend wanting to get back him back in touch with them. Of my mom and her 4 siblings, it was 3 versus 2... three (my mother included) welcomed him back with forgiveness, and a willingness to be civil. My mother admitted after spending some time with him that he was the same person she remembered... self-centered and a bit of a jerk. But the point is that she forgave him.
Two of her siblings shunned him. They're full of anger, and hatred, and they wind up creating tension and drama whenever the topic of him comes up. And what's worse, they've turned their own anger and resentment, and pushed it on to their children. My cousins are irrationally angry at this man they've never met because their mother convinced them they should be.
I suppose my point is... even if this guy's a jerk...which it sounds like he is... I know that you have in you the strength to be the bigger person. The strength to stand up against other peoples' opinions of him, and decide that you're going to be civil. You've never met him, you're just going by what your parents say, and HEY... they're probably right about him. But before you make up your mind entirely to LOATHE the man for taking sides when he shouldn't've... be the bigger person, the way your sister was to her mom.
IF this man is civil to you, I would absolutely try to be civil in reverse. Forgiveness is freeing, and holding all that anger and resentment... it just isn't worth it.
(Take it or leave it, of course, that's just all my opinion.

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