if we put social norms and taboos aside, and leave only what is normal for a given family, then the girls and MC had to be the parents of at least two or even three wonderful beautiful children, since the norms of this unusual family require the birth of children starting from age of 15.
Now they are 18-19 years old and they are still carelessly wasting their youth.
grandmother (Melody) gave birth at 15, mother (Eva) and aunt (Tina) gave birth at about the same age as Melody.
but why did mother and aunt deviate from family tradition and did not insist that their daughters Cisnei and Ruby have children from MC at the age of 15?
Why didn’t the grandma contribute anything to her grandchildren? What's happened?
I'm guessing Grandpa was dead by then.
It's my theory that due to the Grandfather having the same illness as the Protagonist that he's the real father of the MC and Cisney and possibly Ruby as well.
Tina had Ruby after. She's a year younger than Cisney and the MC. Tina is also older than
Eve.
I'm guessing grandad's relationship with Eve is what caused the rift between Melody and Eve, either because of the relationship or her having left the family with the MC's "father" and abandoning Grandad.
What we know:
We know Ruby had a loving father.
We know Ruby's father is dead.
We know MC and his "father" had a terrible relationship.
We know MC's and Cisney's father is dead.
We know Eve hates incest.
We know Eve's father is dead.
We know that people with the disease are attracted only to their family.
We know that Melody had four kids.
We know that Alex and Alison are the Protagonist's, Cisney's and Ruby's cousins.
We know that the disease trigger is stress.
We know that Alex's and Alison's parents had the disease.
We know that Alex's and Alison's mother was Eve's twin sister.
We know that Alex's and Alison's mother is dead.
We know the Grandfather is the only one who lived longer than 10 years with the disease.
We know Melody disapproved of incest (before possibly entering into the Protagonist's incest harem.)
We know Alex is asexual.
We know the only male Cisney has ever been attracted to is her brother.
We know that Cisney's attraction to her brother lasted throughout their teenage years. This is why Cisney avoided her brother.
What we can surmise:
Melody and Granddad were related because they were attracted to each other. So Melody is the sister/daughter/niece/first, second or third cousin of Granddad (possibly once removed.)
Alex's and Alison's father was Tina's and Eve's brother.
Alex's and Alison's father is dead.
The protagonist and Cisney's father wasn't their real father or else Cisney would have been sexually attracted to him.
What we can speculate:
Regarding Eve there are 3 distinct possiblities.
1. Eve slept with her father.
The stress of leaving the family and being pregnant probably triggered the disease in Eve. Therefore her marriage was probably loveless and troubled. Maybe before Eve got entangled with her father she and her husband had been boyfriend and girlfriend, but after the affair the disease could have triggered in Eve and she would have lost all libido for her husband. Either this abandonment or the affair itself caused Melody's resentment of Eve.
a. This would make MC and Cisney Eve's brother and sister along with being her son and daughter.
b. Tina would be their half-sister in addition to being their aunt.
c. Ruby would be their niece as well as their cousin.
2. Eve slept with her brother. The affair between the brother and both Eve and her twin could have broken the family and caused Melody to disown her other two children. I think this less likely because although Melody is actively hurtful and belittling to her daughter, she at least acknowledges Eve as such.
a. Eve is both mother and aunt to Cisney and the MC.
b. Alex is their half-brother
c. Alison is theri half-sister
(Note: If Eve's twin sister happens to be identical then Alex and Alison are for all intents and purposes full siblings.)
3. The MC's 'father' is his father. Eve never commited incest and her dislike of it is based on the familial relationship between her parents and the affair between her brother and sister. This seems the most unlikely as Cisney never expressed an interest in her father.
Regarding Tina there are 3 possiblities
1. She stayed around with her father and had Ruby with him a year later, helping out Melody with the illness. Because of this she's Melody's preferred daughter and the one she helps out. Eve had Cisney and the MC with her father.
a. Eve is the Protagonist and Cisney's mother and sister.
b. Tina is their aunt and sister.
c. Ruby is their cousin and sister.
2. As above but Eve slept with her brother.
a. Eve is mother and aunt
b. Tina is a double aunt
c. Ruby is a double cousin.
d. Alex is their half-brother (see note above)
e. Alison is their half-sister (see note above)
3. She didn't have sex with her dad. She didn't fuck her brother. The illness hadn't triggered in her yet. She married her husband and had Ruby and the illness triggered possibly much later due to the deaths of her father and husband. Eve slept with their father. No matter who the father of the Protagonist and Cisney are, the relationships are unchanged.