You don't fucking say.
From what i observed the relations between slaves are pretty primitive and boil down to if they work one job together they somehow interact and somehow that results in them starting to like each other or dislike each other.
Something along those lines. Haven't yet seen any proof that their relationship impacts anything in any meaningful way or a way that mattered for you soo... like, who cares? Maybe he'll improve this in the second game.
I think i've seen one scene though where their relationship actually for once shown some effect(probably), like there was this gal that was bullied outside my manor and there was this another girl who rushed to her help.. probably due to their friendship?
I honestly have no fucking idea lol. xD