Well, while it is just in the beginning stage and does not have a cross
Ten Less Than Then by L.A. Riverburn certainly qualifies as Ranma's grandmother changing his life entirely.
I thought the story has some decent possibilities, but right now all that exists is essentially a prologue. If it ever picks up again, it promises to be interesting.
I'm blanking on some others, but I know I've read some stories where Ranma's grandmother gets involved. Of them, Siren's Calling and Chaotic Rebirth are definitely the standouts, which were already mentioned.
Oh, in a way, you could possibly consider
Ranma, Mage of Darkness to fit. It's a SM cross by Historianofthekais in which Ranma's umpteen great grandmother is Kasumi (I think, been a long time since I read it). Granted, she doesn't do all that much in his life, but she's definitely present.
And finally,
Stars in the Sky by Chaos Lord has Ranma meet his grandmother and she definitely will have an influence, but it is just getting started. This story is a pretty unique take on the standard SM cross.