How much of an age difference is too much?

[Anonymous]

My brother married a woman ten years his senior. He's 47, she's 57. They've been together for a very long time, but I've noticedthat as the years go by, the age difference is more obvious...he hasn't aged, she has.

My sister in law is just a few years younger than my MIL. Weird. My SIL's age is also supposed to be a carefully guarded secret, too. They adopted years ago and SIL will not tell her own child her age...