Personally I don't see the big deal with a family member being able to see your FF#. My wife, and daughter when she was young enough to share in my travel benefits, have my sign-in info for the non-revenue travel site and I don't give it a second thought. If your family member likes you enough to pay for your ticket, I'd doubt that they'd use up your FF miles booking award tickets for themselves or something like that. Is it that you don't want them to know that you could have gotten a free ticket instead of asking them to buy you one or something like that?

Jim