Issues with guest list

The reality is that you can invite/exclude anyone you like because it's your wedding. The reality is also that you will almost certainly get complaints about your guest list from someone at some point, regardless of who you do or don't invite.

Inviting people in groups is likely to reduce the number of complaints you get, but since 'all adult siblings' and 'all adult cousins' ARE whole groups anyway, I think you are fine to exclude the under-18s. There is a possibility that those with younger children may not come if they are unwilling/unable to find childcare, but that is fine. Just as you not inviting their children doesn't mean you don't care about them, neither does them not coming mean they don't care about you.

We had a smallish wedding of 50 guests so had a blanket no children invited to our wedding. People were fine with this.

Umm going against the grain here but not inviting siblings, I think long term will damage your relationship with them (up to you if that matters but I would always advocate for trying to build those relationships especially as the older party). I say this as some not inviting children as a general rule but exceptions are being made for parents nursing very little ones and family travelling from abroad. If either of us had under 18 siblings they would be invited. As teens they will know they are excluded and the argument of no under 18's doesn't imho hold up for family that close.

We are having a no kids wedding as well. We are looking at it as, we have given them almost a years notice to arrange childcare. So it's down to them to have an issue with it if they've not arranged something when the wedding comes around.

It's your day!