What's a good way to confront my Grandma about this?

It'll be difficult, but maybe sit down with her one day and listen intently. When she's finished, offer your thoughts, and tell her she needs to let go of her frustrations. There's no use in crying over spilled milk, etc... And then remind her how important family is. Family should support one another, family should take the time to work things out, family should just love unconditionally. Sometimes, there are people with families that just don't work that way, but I'd like to think it's always possible. So, bottom line: talk to her, and talk to your mom, as well. It's time they acted like mature adults. Good luck!