My fiancé wants to invite 20 of her ex-boyfriend’s family members to our wedding?

That would be a hard no from me. There is absolutely no reason an ex or ex's family should be invited or in attendance at your wedding. I would sit down with her and calmly explain to her that you aren't comfortable with inviting her ex's family to your wedding especially that many of them.

I once went to a wedding where some grown woman got on the head table microphone and said "although [bride's name] and [her son's name] didn't work out, I'm glad she found someone" type of speech. That's what happens when you don't set and maintain boundaries. Sit down with your FS and decide how you both will live moving forward. Good luck.

No way. That is inappropriate. They need to move on with their lives and it sounds like both of the exes have not done so.

Had we not decided to have a very small destination wedding, I would have invited 20+ of my ex’s family members to our wedding. I don’t understand why on earth you would need to cut people you are close with and consider family, just because you no longer have a romantic relationship with one of their relatives. And expecting someone to do so seems petty, insecure and controlling 🤷🏼‍♀️

It is okay to have some ex-relationship family to the wedding if they have remained friendly, especially having a continuing relationship. Though, 20 people would seem extravagant unless they were the primary people being invited.

My ex husband invited his ex girlfriend and her sister to our wedding. That felt extremely weird, but I didn't want to seem like the controlling future wife. My ex was stuck in his past traumas and dealt with it in unhealthy ways that were extremely and permanently damaging to all those who got close to him and to himself. And I won't go into an explanation, but he would also reminisce old relationships when there was trouble in a current one. In his case, I'd say that it was one of many warning signs and I was young and dumb. But there's other situations where family remains close to a ex mother in law or something like that who treated them like their own. But if they're the type that are going to be constantly mourning the ex and your fiancé together, or always pushing their family member's prerogative, that's where it would be cause for concern. So every situation in my opinion is different, but 20 seems excessive for most situations. I guess it can only be answered on a case by case basis, but it's something you're going to have to work out with your fiancé together.

Is there a reason she feels close enough to the ex and 20 of their relatives to invite them to the wedding? If they genuinely had an amicable, mutual breakup and maintained a healthy, platonic relationship afterwards, I could see viewing the ex and family similarly to a family friend. If there's some "stuck in the past" thing going on, that may be a different story.