I think the relative pronoun in Engadinese will be o (on before vowels), derived from the neuter relative pronoun ὃ. (But the Engadinese one will be invariable.) Since there’s no masculine nominative singular article ὁ for it to conflict with, that should work all right.
I think that i(n) might also be allowed for feminine referents, but this will be archaic or poetical.
I still have to figure out how to mark possession, though. What’s tempting is to do something along the lines of Modern Hebrew, where shel apparently comes from she “relative pronoun” and l- “to”, so A shel B is “A which-is-to B”, or something along those lines. But I’m not sure whether there’s a good pronoun to fuse with the relative that would work.
Maybe use ek as in TAKE?