Actual twins who play twins in TV shows (the Olsen twins, the Sprouse twins in Zach and Cody, etc.). Additionally, is there a trope for when a prank is done to someone, but one of the things that occur is real. Example: Someone does a Halloween prank on a friend, but it turns out one of the ghosts that scared that friend is real)
Topic on Forum:Trope Talk
Regarding the first: Not specifically for that, no. It'd pretty obviously be a subtrope of Real Life Relative, but I suspect casting twins as twins is basically People Sit on Chairs territory. If you want to go ahead and propose such a trope in the Workshop, though, I certainly don't think it'd be dismissed out of hand, and I can give you a few more examples to flesh it out:
- James and Oliver Phelps as the Weasley Twins in the Harry Potter movies
- Tia and Tamera Mowry in Sister, Sister
- Jean and Liz Sagal as Kate and Allison Foster in Double Trouble
- And an inversion when the Sagal twins were cast in Grease 2 -- they auditioned and were hired separately, and when the production team realized they had accidentally hired a pair of twins, they created twin characters for them.
As for the other... It probably comes under Mistaken for An Impostor or one of its related tropes, like Your Costume Needs Work. Likely depends on how it's played.