I thought about this topic.
If not for one but -
1 It is obvious that Katie and Amber are not like each other like two drops of water. They are not twins. Therefore, they cannot be conceived at the same time.
2 or - they are sisters who shared one uterus - they are from two different eggs that somehow found themselves in the same uterus
Their mother is born from mixing races. Mother carries genes in both white and dark person in eggs. And it could turn out that of two eggs one egg gave a more white girl and another egg gave a darker girl.
... Such a probability of course exists.
3 In addition, the date of birth is not indicated in the info about the characters.
I allow the option of superfetation. They could be born in one year but on different dates for one year.
So - what is the closest version to the truth?
And another factor that says that they cannot be one-egged twins - they have different skin textures. Ambert has as if plastic skin - that is, her skin is rather oily, glistens, and darker, while the skin of Katie is more dry and more pale. There are no external signs of twins with such a set. That is, it seems to me a more real version of superfetation and that their mother was a “walking woman” - during pregnancy, she became pregnant from another man than the one who was a father either Katie or Ambber.
about same birthdate - in the character info sheet only age - 34 years. and only.
I can re -run VN from the start.
Perhaps I forgot some dialogue where they tell each other about the date of birth, but this will not change the fact that the skin color of two sisters varies.
That is, if you do not know about the possibility of superfucation or about the possibility of neighborhood in the uterus, then the idea that sisters may have one date of birth seems to be a complete nonsence.
Let's keep this simple: they are fraternal twins, and they have a brother who was born with them. Fraternal triplets. 3 eggs, that became 3 zygotes, that became 3 babies, and so on. They don't have to look anything alike, and they can still have the same two parents. I worked with a woman with a twin brother. She was pale skinned with curly blond hair, his skin was a bit darker with straight black hair. A family photo revealed they were clearly a mix of both parents. My sister and I look a lot alike, enough that her friends used to ask if I was her twin. I'm 3 years older.
So for the hints Ricoh was dropping: MC was raised by their father, though their father occasionally visited Katie. MC thinks his step-mother is his mother, or at least hasn't revealed anything otherwise. Katie was raised by their mother, but I imagine she had no love for the other two. Their father must have decided he was going to be the one to raise his son to make him a man. Their mother took the girls, and this part is speculation, but I'd bet she kept sweet Katie while throwing out fussy Amber with the bathwater. It could also be that their father took Amber, and he gave her up, but it will still show that both parents have their chosen favorites. No matter how the story goes, if the triplet's parents are brought in, there will be drama. Hilarious drama.
What I look forward to with the in game reveal is Emily's reaction. Looking at her parents, realizing their relation, putting her hands to her temples, and saying something like "Oh my god, Aunt Mom and Uncle Dad!?" Katie would probably faint. Amber would get turned on.