Guest list

We’re only having max 50 guests in the day time and that consists of : immediate family, aunts & uncles and close friends. One of my aunts passed away and on that side of the family only my cousins are left. No cousins are invited to the day time ceremony except one which I want as a representative for my aunt who died. How do I go about explaining this to their sibling? It’s just a rule we have for budget and family otherwise the costs and lists would be huge.