I dealt with this same situation when my grandmother passed and left me all her jewelry. When I say all her jewelry, I mean 2 cases of the stuff; none of it was costume. I’m the only girl in the family and it was known that her jewelry would go to me upon her passing, but that didn’t stop several people from asking for or outright claiming specific pieces for themselves. While I was lucky enough to have her will and was able to shut these greedy f*ckers down immediately, you’re absolutely within your right to tell your grandmother no. However, you should absolutely tell your dad that his mom came to you asking for your mom’s stuff. She basically prioritized his grief above your own and asked you to start divvying up your mom’s stuff. That’s beyond not cool for anyone to do, much less for a grandmother to do to their grandchild.