I [i]do[/i] like math, so, I present options for easiness.

[u][b]Option 1:[/b][/u] Only the middle and youngest child are Sabrina's.
The youngest Sabrina realistically could be in this case is 37/38.
The middle child would be Sabrina's oldest; 17/18 years old. (Sabrina got pregnant at 20)
Which would make Cosette 16/17, born about a year later.

My character would be older, from Thierry's previous situationship. Like 20 or something. It would mean Thierry got her at 19, with another woman. #unplanned #oopsie

[u][b]Option 2:[/b][/u] All kids are Sabrina's.
The youngest Sabrina realistically could be in this case is 39/40.
My character would be Sabrina's oldest; 19/20 years old. (Sabrina got pregnant at 20)
Middle 17/18 and Cosette 16/17.


Personally I like option 1 the most, brings in extra drama for the rebel oldest, and more reason for her to make Thierry's life hard. 

The moral of the story is that Thierry is bad at birth control. Especially if he knocked up a girl at 19 himself, to get my character from the previous situationship, and 20 with Sabrina. :lol