[Texasred]

I was in a relationship much like this one when I was in my early 40s: my girlfriend was 17 years younger than me. We hung out with her friends, with my friends, nobody cared, not even her mom who was I imagine much closer to my age than my gf was. The gf was a little wild - she called me up and asked me out the first time! - and not as ready for a serious relationship as I was, so I moved on. Imagine my surprise when she found me on LinkedIn a couple years ago; we've gotten back together and the age difference seems non-existent.
I would say to your sister, it's not the age that should matter but whether or not you have interests in common. And of course, health; I was in excellent health then and am now, but living with someone much older in poor health could be an issue.