Quoting Vodka Knockers:" That's actually a pretty boss way of saying it. She knows all too well about the things that's went on ... [snip!] ... it wasn't this way and I could go down there and everyone get together and have a good time, but...Just doesn't fall that way."
Yeah, that sucks =/ Sorry you have to deal with someone like that.
Definitely just be sure to remind her you're excited about your trip and almost coddle her in a way, lol. She'll end up focusing on the fact that you're coming for a visit more than the fact that you're asking for her son to not be there.
Worst comes to worst she'll complain about it and you'll have to put your foot down and say, "Either he's not there or I'm not there." Hopefully that won't be the case though, especially if she's wanting to see her grandchild. Good luck.