I thought meeting the Fuddy family was verboten, next thing she will be leaving her toothbrush at your gaff and picking out cushions.
Does he know that you only see his sister so that you can get your hole? Or has she perhaps told him it's "serious"?
You could bring it up over Christmas dinner. "May I say this turkey is superb, by the way, your sister bangs like a saloon door when she's got a few G&T's in her and who knew to look at her that she loves it up the Gary? Any more sprouts?"