Yeah, I think many of us have seen the same kind of thing happen. My sister used to have a huge crush on my best friend up until she found her current guy, and obviously my friend and I get along so well in part because we share a lot of the same traits and values, so I always found it kind of funny.
Also, one of the funniest things about growing up with my best friend was the fact that many of his friends thought his sister was really hot. The hilarious bit is that there is a really, really strong family resemblance between the two, and they looked even more alike before puberty. To this day, I cannot help but think that his sister looks just like him, except with longer hair and boobs tacked on, so it hits me as kind of funny when guys drool over her. He doesn't find it quite so amusing, for obvious reasons.