Him being illegitimate doesn't make sense here since both the MC and Yuzu are related to the uncle and aunt by their father's side, so he and Yuzu share equal amount of blood relation to them
Let me guess, this will be Blue Balls™ the manga. He will never get far enough to having sex and since exp gives pleasure he will be akin to an inanimate object like a vibrator, getting nothing in return.
This is just Netsuzou Trap, "not yuri" my ass. And even then the male characters in Netsuzou Trap had actual plotlines, in this "MC" is just a plot device for NTR fetish