Apparently you can set a relationship of Megan from mc's teacher to something else, not sure about mother since she does look young, but maybe she just looks young (sorry, played this a while back and never got to unlock her character). Also you can change Sarah from mc's bestfriend to maybe sister(?). Now, I don't know when you change the settings you are basically pretending that they are your family or if the actual dialog changes. If we're just pretending they are family that's pretty pathetic, unless the dev wrote their lines in a way that works both ways mother/teacher and sister/bestfriend. Again, I never got that far to see how their dialogs are.
Afair, Megan and Sarah's ages are specified in one of the flashbacks at the start of the game and if we set Megan as MC's mother and Sarah as MC's sister, then this would mean that pedophilia in WaW is ok, because MC's father is not in jail for impregnating Megan two times in her 10-13 years old (don't remember Megan's exact age, but it's something around 32).
And no, dialogue does not change when we change Megan/Sarah' roles. This game does not written with incest in mind. Dev added the ability to change roles for an easy way to get the "incest" tag.
Edit: Ok, so in the first flashback Megan is 34, Sarah is 22, MC is 18. If Megan is MC's and Sarah's mother and Sarah is MC' sister, then Megan gave birth to Sarah at age 12 and to MC at age 16.