A few ideas that would sound nice on acoustic guitar:
“All I Want Is You” - U2
“Crazy Love” or “Tupelo Honey” - Van Morrison
“Fields of Gold” - Sting
“Wonderful Tonight” - Clapton
“In My Life” - Beatles (this is more of a father/daughter song)
Dunno how these would work as guitar duets, but as wedding dances they are hard to top…
This Will Be Our Year is an awesome first dance wedding song. I think it would work as a guitar arrangement.
Something a little more contemporary that I’ve seen that works well is “Such Great Heights.”
And here’s the guitar-based cover by Iron & Wine. It works well with male & female harmonization, too (if you plan to have both father and sis sing.)