Wow. I did not realize this has been going on for 10 months.
I would go to the party. Talk about the weather or something else. You don’t have to get into about the dog or the litany of things people could have done better by your wife over the years.
Did your wife reach out to her and she was rebuffed or is your wife just giving sis the silent treatment and waiting for an apology that may never come?
I have BTDT, made it work in front of our parents and I’m happy that I did. Average life expectancy for men in the US is 73.5, so Dad is already over that. Could be his last birthday or he could have 20 more. Who knows?