Is the questioner's inability to conceive until now due to her not fulfilling the vow that she and her sister made, especially after both of them had miscarriages?

The majority of scholars are of the opinion that fulfilling a vow is not immediately obligatory, and it is permissible to delay it. There is no necessary connection between delaying or abandoning the fulfillment of a vow and what occurred, such as a miscarriage or a delayed pregnancy. If the vow contingent on pregnancy was intended for the birth of a child, not merely the pregnancy, then it is not obligatory to fulfill it if the pregnancy miscarried, because what matters in vows is the intention of the one who made the vow.