Is it obligatory to make up the fasting days of a month vowed for the recovery of a sister, given that three years have passed since her recovery and the one who made the vow is seven months pregnant, or is expiation permissible instead?

A vow contingent on a condition is disliked (makruh), but it must be fulfilled if the condition is met, according to the hadith: "Whoever vows to obey Allah, let him obey Him."

You must fast the vowed month when you are able. Immediacy and continuity are not required according to the majority of scholars. You are not permitted to resort to feeding (the poor) as long as you are able to fulfill the vow.