Btw, I just realized you can set relations with the Markus Letter. Is this simple word replacement that doesn't really make sense with the plot, or is there a legit half sister narrative?
Both options make sense.
Your mother was a noblewoman married to a nobleman she did not like.
Markus was her bodyguard, who was fucking her, and the two were in love.
Family fell, mother and her husband died. Markus escaped with you and hid in a small village.
(where he tried to form a new rebellion group... before those were taken over by a demonic werewolf cult)
Markus fucked the widowed neighbor woman and impregnated her with Mira, your neighbor/childhood-friend/stepsister OR sister
The letter just determines whether your biological father is markus or your mother's unnamed cuck husband. (both of which were having sex with your mother regularly and thus could have been your father)
If markus is your biological father, than Mira is your blood sister, and also your neighbor-childhood-friend
If markus is not your biological father, then mira is the secret daughter of your adoptive father. (I am actually not too sure if step sister counts in this case)
By default your biological father is your mother's cuck husband, not markus. But while this was meant to avoid patreon rules against incest, the fact is that the story
actually works if you leave it on the default. Unlike all those "landlady/landboy" relationships which are blatantly nonsensical. In this game the relationships are fully functional either way