Personally... I would agree with you on this.  It's her uncle... mom's brother.  It will be a chance to see other relatives too.  Maybe he can negotiate a concession on extra time another day? 

I hope your husband never needs a concession from BM, because he very well might not get it, and frankly wouldn't deserve it.  Unless BM has a history of being uncooperative, I don't think what she's asking for is unreasonable.  A kindergartner can miss a day of school without a problem, and he can negotiate with BM for some extra time (preferably before November).  BM isn't asking for the time to do something frivolous, it's her brother's wedding!  

If BM and your husband do not co-parent well to start with, then maybe it's not a big deal, but if they do, IMO your husband is going to damage the co-parenting relationship by saying no.  I think the saving grace for my SO and me where the kids/BM are concerned is that he and BM *do* co-parent well.  They willingly exchange days without being too tit-for-tat about the time coming out perfectly even.  That enables the kids to attend special functions on both sides of the family without worrying about whose day it is, it allows the adults to go on vacation without the kids and have the other parent cover the time, and it allows each parent to take the kids on vacation knowing that the other parent will cooperate if a few of their days are affected.  Your husband and his ex might not have that kind of relationship, but I hope they at least have one where there is some flexibility.  This is a time for flexibility.

It's her uncle. It's family. DH and BM divorced. SD didn't divorce anyone; she's still a part of both families. It's a family wedding. She should be there. Whether it's Kindergarten or grade 12, missing one day of school for a family wedding (or funeral) should be acceptable.

DH needs to rethink his stance. Otherwise BM will never allow SD to attend any of the family things that occur on her parental time. He can ask for a makeup day if he wants.  BM should be willing to give him a makeup day. 

We set and enforced a ZERO missed school boundary.  The CO was clear.  Visitation began and ended on a schedule that complied with the school schedule.  

The sperm clan tried the miss school crap periodically and we would smack them with a rolled up copy of the CO when they did.

If the uncle in this case wanted the SKid at the wedding he would have scheduled his wedding accordingly.  The SpermIdiot’s sister tried to pull this crap for her wedding to no avail.  We told them to pick a date that did not conflct with SS’s school schedule.  They did..... then they accused us  of purposely ruining her wedding when the SKid broke his arm 2 days before traveling to SpermLand for visitation and the wedding.  He picked a bright red cast which clashed with her wedding colors.  As usual we just told them to F-off and grow up.  4yo kids sometimes break their arms and sometimes pick a red cast. The cast color had nothing to do with anything but his personal choice for cast color.

I'd think something like this:

We make one concession, they make one concession. As long as it hasn't been a problem in the past and doesn't get out of hand in the future, this is all fine.

I'd say something like this:

"I don't see anything wrong with that, at all. In fact I think it'd be SUPER. Let me discuss with hunka-hunka-hubba-hubby to see if we can make that work. Ultimately, it's up to him, but I do have some sway (wink, wink)." Then go to him and ask him what his concerns are and try to find ways to help facilitate.

If he flat out says, "Yeah, I wanted the same thing once, and she shut me down, COLD. Wouldn't budge. So let's call this karma," I'd say, "No, karma is what happens - not what someone does - which is revenge. Let's eschew this, but maybe gently mention that past incident, and say we remember how crappy it was when cooperation didn't go well - and that's why we won't do it to you, but most importantly, to dear sweetiepie. We'll make an exception this time."

Not fully aware of your situation, on the surface, I'd say it seems like if he lightened up this time, it'd go a LONG WAY to making your world a better place.