We have two "sisters", Clem and Goose,(not genetic, but only 6 months apart) who go through phases of being inseparable.
The absolute cutest is when they go to eat at their side-by-side food dishes together, and twist their tails together like they're "holding hands". They go up and down the stairs together, looks kind of like cattle herding.
One of these two, Clementine, follows each of her two sisters to the litter box because she has to make sure they cover up. If they don't, she will. This is a good thing, since the third sister, Winnie, is pretty lazy about that.