MB has already said, a long time ago, that the whole family will get pregnant, yes, even Allison. Pryia's treatments work as the plot armor (alongside MC's Super Sperm) to allow the older girls to get pregnant. While rare and potentially dangerous to the mother to be, pregnancies of women above 60 years old, exist.
The M7 will ALL be pregnant, without option. Some may even already be. In those cases, you will not be asked or informed (AFAIK) except, maybe, on the Relationship pages.
Mabel and Ophelia will be optionally impregnable. I can't even imagine the nightmarish number of renders per scene with all the optional pregnancies. Stay sane MB, stay sane.
Peace