I don’t know background as to why sister didn’t want to even speak or see your dad, but I’m struggling to understand why that was needed- it’s a pretty nuclear option to effectively set an exclusion zone around herself.
i was in same situation as your sister. Nephews weddings . My dad invited. Him and I were not in contact for 20 years. My view was that it was nephews day and if my dad approached me I would not do anything other than polite niceties like I’d do with any one I was meeting for first time. If dad tried to engage further or make any attempt to raise our relationships first response would be to excuse myself to powder my nose, if he persisted to say clearly it was nephews day, I wanted to enjoy his day, and that if dad wanted to tlak about our history and catch up on my news we would discuss that after the wedding at a different time. My third option was then my other sibling (not father of groom) to firmly direct him away. But I wasn’t going to ignore dad, or refuse to acknowledge or speak to him. I was his daughter, and fankly it would be a bit unrealistic to expect him to pretend I was invisible.
I also made it clear to other sibling, that my sons (his gc who’d he’d not seen for 20 years) were interested in meeting him. And that he could direct dads attention to them
and that’s what happened. We said hello, how are you, and I drifted politely apart to let my sons engage with conversation and that kept dad more than happy.
it’s way too late to do anything about this now. My point is I think your sisters approach was unrealistic and bound to create an issue. She could have just been icily polite and grey rocked him so he’d get bored and moved on. She was frankly making a point at him and creating a reaction form him by her exclusion zone stance. So, not your issue. That’s on your sister.
if the issue for your sisters approach of an exclusion zone was becuase your dad was abusive to her, then frankly wtf ? Why was he even invited? He simply should not have been invited if that was case, and it was cruel to do that to sister.