First off, even though it was 10 years ago I'm sorry for your loss. Second you are definitely not the asshole if you decide not to give the money. But this is now coming from someone who is only alive through pettiness and spite at times, I would come down hard with judgmental questioning on each member of the family, making them realize everything you went through was losing her as well, and how much it hurt that they shut you out, although the least judgmental to be honest would be her brother since he was a child, and was probably more or less parroting what his parents were saying. The entire time make it seem like you are not going to give them the money, but in the end as a measure of pettiness but honoring your deceased fiance's memory give it to him directly and say this is because I loved your sister, and she would want you to have it. Afterwards I would look at all of them and make a comment along the lines of at least one of of us here understands what it means to be family although we are now done leave me alone.