Have you given Arch a try yet? Their are also some family friendly projects like Antergos. Arch wiki is my all time favorite for great documentation, even when not using arch, go figure.
It's a very educational experience, but not really something the "average grandmother" is likely to have the patience for, particularly when they're just trying to watch a flash video on Facebook. That's the real strength of Mint, and I'm not sure what best fills that niche if Mint is off the table.
EDIT Write the post install script yourself don't use one of the many flawed ones that a search will show up.
I really don't see why the downvote? The OP wants an easy to use distro that their grandmother can use but seems happy to install it them-self.