OK, this might be a gender thing, but many, many brides would freak at being asked to add 5 young boys to a sit-down engagement party with what ... 24 hours notice? 48?
Are there going to be any other kids there? Were your boys even invited?
Many many healthy loving families leave kids home for engagement parties. It's your choice to go or not, but I don't think not being able to add the boys says anything about your sister's character.
As for the exchange with your wife, I like the way you handled it. Great job!
(You might want to address Canadian Thanksgiving as well as Canada Day specifically in your custody agreement, along with US holidays, because I can see this being an issue every year.)
Me: 44 H: 44 Kids: 20, 16, 16, and 10 Together/Married: 22 years H announced he was emotionally detached and considering D: 4/4/16 H announced he is going to try to stay and reconnect: 5/1/16