Obviously Mami is going to try to pull something here, but at this point in the story what can she really do? (in a believable way anyway). My mind is a little fried, but I'm pretty sure Kazuya's grandma already knows Chizuru and Kazuya aren't dating, so that's out the window. Considering Ruka is already the candidate to change grandma's mind on who Kazuya should date, I just don't see what Mami can realistically add to the story at this point.
I mean, i don't think she'll do something too bad, worst case scenario, she exposes Kaguya and Mizuhara to the entire school which i don't think will happen, Grandma wouldn't be that rude to both of them when she finds out (she's definitely gonna find out) unless Mami literally commits a crime, i don't see a worse scenario