Or it could just be that The idea that started it all was about Mom, Aunt and Teacher ad since the sisters were there might as well have sex with them too because they have to be there anyway and why not?This may be a dumb theory but I think one of the reasons why the game is called Milfy City is because maybe if ICSTOR wants to put impregnation/breeding/pregnancy into the game then maybe you can have all of the female characters get pregnant. Now the reason why I came up with this is because like many words and slangs the meaning of the word milf has changed a lot over the years.
The first part of any game is to create all the characters you will deal with everyday.
MILFs include - Mom, Aunt, Teacher, Neighbor, and even the Twins are Older than the MC.
Just because most of the sex so far in a V2 development involve your sisters there are equal amounts of sex with Mom and Teacher with plenty more to come is those Work In progresses go away in the next release.