That one's hard to top on sheer ick factor because it combines a serious taboo with something just plain unappealing in the first place. While grandmas aren't necessarily super old these days, the image you get when someone says granny is still a wrinkled old lady with white hair. Just like when someone says grandpa, I think of someone who looks like Hugh Heffner. Ew. You combine that with incest, and you can't really get any grosser without involving excrement or animals.
You can do meaner, you can do funnier, but yuckier is gonna be hard.