"The First Man You Remember" would be a touching choice. "I Don't Need Anything But You" from Annie is nice too.
Seeing as you are related a love duet might have too much of an ick factor.
"Old Fashioned Wedding" could be cute, but maybe change some pronouns first. You could be asking your grandfather to foot the bill for your fancy wedding and sing her part as written. He could sing "You'll have an Old Fashioned Wedding" instead of "We'll"/"He'll vow" instead of "I'll vow", etc.
Marie: Don't be in such a hurry about that pretty little chippy in Frisco.
Tony: Eh, she's a no chip!