We said no kids at our small wedding and had to compromise for family traveling across the country. Are we being unreasonable for not compromising aga

My fiance and I are getting married in January in the mountains, it's a 2 night stay at our venue (the night before the wedding and the night of the wedding), an evening wedding, and then our reception which consists of a 2 hour dinner service and then a very low key night of socializing and getting to know each other (since our families live across the country from one another and haven't had many chances to spend time together). Our guest list will be less than 20 people.


Because of the set up of our event, my fiance and I decided on a 'child free' wedding. For the record, inviting the kids would add 6 children (ages 4, 6, 7, 8, and two 11 year olds). My fiance's brother and wife will be traveling across the country and are hoping to stay a few days after our wedding, so when they asked about bringing their kids to accomplish this, we said of course. We wouldn't expect them to leave their kids (ages 7, 11) home for 10 days while they were out here visiting. My fiance's cousin (lives in the same city as us) with children (ages 4 and 6) was happy to find care for the kids for the two nights of our wedding, no issues there.
My fiance's sister (38), on the other hand, has thrown us a bit of a curveball. Her boyfriend (41) of one year has two girls (8 and 11) that we know quite well and she felt that if her brother's kids were coming (from across the country) that the girls should be allowed to come too (they live in the same city as us - 1 hour from the venue). We politely reminded them that we were aiming for a child free wedding due to the nature of our event and the venue and that the brother's kids were coming more out of necessity than invitation, and said no, unfortunately the girls were still not invited.
A few weeks passed and my FSIL brought it up again to my fiance (I wasn't there), saying it would be good if we could make up our minds about having the girls at the wedding so they could figure out plans with her boyfriend's ex-wife about which days they would have the girls over the holidays to accommodate the wedding and custody arrangements. My fiance again said that the girls weren't invited and this time, my FSIL seemed to understand. She asked if instead we would consider having a shower for the girls to attend and participate in, since they wouldn't be at the wedding. My fiance agreed that a shower would be fun and having the kids at the shower would be a great idea. That seemed to be the end of the conversation.
A few weeks later, my FSIL once again approached my fiance and asked if the girls could come, this time to the ceremony only and then not attend the reception (her idea was that the ex-wife would bring the girls for the ceremony and take them home right after). My fiance told my FSIL that we would talk about it and get back to her. We're feeling stuck. Our ceremony will be less than 15 minutes, and it seems a long way to drive for 15 minutes and then have to drive home in the dark, in the winter. Plus, we've already said no more than once. We also aren't making this exception for the cousin's kids. We keep saying no and they keep coming back with "compromises".
We're not trying to be rude or exclude anyone, we just feel that the venue and the event in general aren't right for kids, and we wanted to have a weekend where adults could relax and connect and not worry about keeping an eye on the kids 24/7.
Any advice for us going into our next conversation with my FSIL? Do you think we should stand firm or just allow the girls to come to avoid the drama even though it's not what we want?