I don't think I've ever been mistaken for a relative of a partner, but through the later part of my teen years my mother and I were mistaken for partners more than once. This was partly because of the places that we were at the time.

I find it hilarious now. At the time I think I was more of the 'can't you tell she's too old for me you weirdos'*?

* I'm hard to pick an age on. I spent the years 14-34 being mistaken for mid-20s.