Valerie was Gaylord's wife, who essentially became a sort of adopted mother. She took care of the MC for a brief period as a child before Gaylord turned the MC over to the church. This event and her relationship/love for the MC and MCs Mother was a large cataclyst for Valerie's slow decline to the state you first find her in. She was close friends with the MC's birth mother and took the MC on as her responsibility/son. Eva and Mirella are essentially siblings via adoption (through the MCs eyes, but not in any official capacaity). At least, this is how I am remembering things. You're right - there are a lot of characters and this game has been in development for long enough that I may be mixing up some of the details.
Edit: the church was
technically a boarding school and I changed out "step" for "adopted" above (but again, I don't recall it being official/legal).