If I had a nickel for every time we have been asked "Are they twins?" or "Are they brothers?" ...I would be RICH. Really, really, really rich.
I answer with some variation of "No, they are not twins, but they are most definitely brothers!!"
Sometimes I get, "but are they real brothers?" followed up with "you know what I mean"...to which I replied, "No, I don't" and keep walking.