When may a sister fast three consecutive months to fulfill her vow, knowing that she has recovered and given birth to a baby girl, and is now breastfeeding for two years?

Your sister must fulfill her vow by fasting for three consecutive months. It is preferable for her to begin as soon as she is able, choosing a time when she can fast without interruption from obligatory non-fasting days like the two Eids and the Days of Tashreeq. Menstruation does not break the sequence; rather, she should resume fasting afterward. Also, the fasting must be completed before the beginning of the month of Ramadan (definition).