Translation:The man who is sitting there is my dad.
No, vem cannot be a relative pronoun in Swedish. You wanna go with som which can cover a lot of English relative pronouns:
- The man who’s sitting there. = Mannen som sitter där.
- The car which is red. = Bilen som är röd.
- Pick the one that is blue. = Välj den som är blå.
I have a quibble with the tense. Swedish doesn't have this distinction, but if we say "The man who sits there," we mean the man who usually occupies that place, unless we're being ultra-formal and archaic. If we are looking at a man now in a chair, we would always say "The man [who is] sitting there . . . " So I am suggesting that should be the main translation.