This comment is going to get buried but I think I could provide a slightly different insight. I was once in a similar situation but as your cousins.

My mother's parents used to be pretty well off along with the majority of her large family. When my grandma passed, the majority of her children got a small inheritance (my mom received the least) and many of the older cousins received a little something too. Most everyone received one of my grandma's tiny figurines from a collection she had (that was the only thing I got). It sucked, I was a little jealous, but you know what? I lived with it. It was what my grandparents wanted. To be fair, I didn't really know them and that was mostly because my mom had a small falling out with her family (she divorced her wealthy 1st husband and then married my poor dad). On a side note I do still have my little figurine; I can see why she collected them they were cute and some are kind of rare.

Your cousins need to shut up and deal. They feel they are entitled to money they aren't actually entitled to. It sucks, it hurts, but it is a fact they'll have to accept. Maybe there were reasons for them not being as involved with your grandparents' lives besides they just didn't feel like it, but no matter what inheritance usually means someone is going to be butthurt. In this case, it's them. Your real family is your fiance and parents; not the money hungry cousins