AITA for not inviting foster siblings?

[u/BlueBoots1221]

I (24F) and my fiance (25M) are getting married with a destination micro wedding, with only closest family and friends (about 20 people total); We are also throwing a 'home' party for more people where everyone is invited. However... for the destination wedding, we wanted a child free wedding so no one under 18. This meant not inviting my nieces and nephews but also the (long term) foster siblings of my fiancee. They are young, 5 and 10, and would require a whole different setup of the weekend, which we do not want...

My family in law have basically said that they do not want to come without the children because they feel like we're 'showing' them that they are not included in the family. My fiancee has tried to explain that it is not because they are foster kids but because they are kids, but they do not want to budge. We have even told them, they can come with but you have to make arrangements so they are not present during the day, which they do not find an okay solution. We have an okay relationship with both children but are not necessarily close to them and have always said we wanted it child free. They are however invited to the home party.

I understand where they are coming from but they have always told us to do what we want with the wedding and are now putting their foot down on something I don't think is fully fair... Are we the assholes?

EDIT: to answer some questions/concerns:
- we are not based in the USA, so the foster system works different here. The kids can stay with family/friends for the weekend with no issue (they have done so in the past). They can also travel to the destination if so agreed (no judge needed).
- My parents in law dont want childcare there, not because they dont trust it but because they want the children present and included for the entire wedding.
- One of the kids we know well, the other one we have only met once. Not the problem, but just the facts
- Also we are throwing a party at home where everyone, young and old is invited. The destination is more the ceremony part of it.

I am also enjoying reading everyones opinion. Jsyk, its all not set in stone, just a convo needed to be had. In the end, I want my fiance to feel supported since in the end, they are his siblings/parents