Mutual friends and exs and navigating social events

[VIX1307]

I'm curious as to what other people's opinions are on this subject in relation to my sister and her ex.

My sister and her ex (late twenties) dated for nearly two years and suffered a rather unhappy breakup just before xmas. He was manipulative and patronising and basically just a complete dick.
They met through mutual friends. A couple who are recently engaged.
My sister has grown up with our friend-the female of the recently engaged couple (not EXTREMELY close like when we were children, but still friends) but she is one of my best friends so we all socialise together once a month or so in a large group (minus the ex bf as he lives far away)

Her ex has now been a fairly close friend with the groom for about 5 years.

The engagement party is looming for our friends. My sisters ex is going- he ended it with her and basically is over it so for him not an issue.

My sister is refusing to go saying she doesnt want to put herself in the position of seeing him and getting upset and said she just knows she wont be able to hold it in and doesnt want to be in tears in the toilet all night.

Im now stuck between my sister and best friend, who feels as though my sister should suck it up for one night and come for her to celebrate this milestone. We have known each other since birth basically (mums met at antenatal classes) and our whole family and many friends will be there.

My sister on the other hand says she doesnt want anything to do with her ex or for him to see her obviously upset. She will spend the whole night feeling awful and worried it would set her back.

Its caused a rift between them and our friend is upset that my sister is making this about her and not the engagement.

Who do you side with in this situation?

[Hotwaterbottle1]

I think your friend should respect your sisters feelings. If it was her wedding fair enough but to be honest I don't see engagement parties as being a huge deal. I'm sure your sister could send her a lovely card.

[Zumbarunswim]

I think your sister is seeking to avoid making it all about her by doing the sensible thing and not going. I also don't see engagement parties as a big deal, hopefully by the time the wedding comes she will feel better but it's totally unfair to expect her to go to a social event she isn't comfortable with. It's not like she will be the only one speaking to the newly engaged couple either-they will have plenty of people to get round and make pleasantries with.