I kind of like the way I told ds, and you might be able to adapt it to your situation by doing it to your ds in front of your dad.
I really hadn't planned on telling him that day but bean had other ideas and really seemed to be reacting to his presence and wanting him to know about his/hers. ds was standing by the measuring wall and we were trying to figure out if he was six fet yet (he isn't) so I said, comfortingly, "Well, you're taller than your sister, anyway. She's only about three inches tall."
He did a bewildered double-take and glanced towards dd's room, and I said, "No, no, no. Not THAT sister, your OTHER sister!" as if it was painfully obvious what I had meant.
I didn't really get to tell dd because her bf was around when I found out and I kind of wanted her to be the very first person who knew. Then I had to work a bunch of double shifts and her work schedule was crazy too, so we didn't see or talk to each other for about a week except for on Instant Messanger (and I wasn't ABOUT to tell her on that!) and when we were finally in the house at the same time and both awake she said she'd already figured it out from the way I was acting.
WTF? How could she have known I was acting any particular way!
Other than that, I just tell people. I can't believe they don't immediately know by looking at me and ask, but since I am 42 and single and we have a lot of cancer in my neck of the woods, they are probably genuinely afraid that it's a tumor and I can't afford medical care.