Kempis, I don't think they're likely to give you the number. My father has the same name as I do (but he's not a Medallion). I've sprung a few "surprise" upgrades on him in the past, but DL has always demanded that I give them his SkyMiles number -- they wouldn't tell me what it was. In fact, they wouldn't even tell me which flights he was on, so I had to have that info, too.
Fortunately, I'm on good terms with the admin ladies in his department, and they'll give me the information.
If I were in your position, I'd just call Sister up and get the info from her.