Would it cause any confusion in daily conversations if we say su and mean "his" instead of "her" or "your", or vice versa?
Possibly, but it's usually clear by context (people know they need to make it clear when using "su").
Thank you, that cleared it up.
I keep imagining reading a letter that said "you know Tom, his brother is married to your sister"....or your brother is married to his sister, or his brother is married to his sister....
or your brother is married to your sister :DDD
Good point! Maybe they consider all the possibilities and that's why Spanish love stories are so... well, lovely! :-D