Help!

So it turns out that my grandfather's younger sister may still be alive (she's fast approaching 90) and I would love to make contact. I'm prepared to write letters to people listed in the white pages that I can assume are likely relatives, but I have no idea what to say to these people without it sounding like I am after money and/or a new kidney.

Has anyone had any success or failures or tis they would be willing to share?