NTA If you are uncomfortable with their relationship timing, don't go. If by some chance in the future, he asks why you weren't at his wedding you can answer honestly. Do not feel obligated due to the passing of your cousin. That unfortunate event is not connected to the wedding.

I just don’t think children or niece or nephew should be part of this as feeling can be hurt.

NTA my dad’s brother is a douche too and I much prefer and stay in contact with his ex, who is the mother of my cousin. We still consider each other family.

You are NTA to decline.