What is the ruling regarding a woman who vowed to fast for a month and began to do so, but then her fasting was interrupted due to pregnancy and breastfeeding? Should she complete her fasting, feed (the poor), or what should she do?

Scholars have differed regarding whether one who vows to fast for a month is obligated to fast consecutively or not. The more correct opinion is that consecutive fasting is not obligatory. So, if she separates the fasting days, she must fast for thirty days. If your sister fasted for seven days, then there remain twenty-three days that she must fast. It is not permissible for her to feed the poor instead of the obligatory fasting; rather, she must complete the fasting of her vow once her impediment is removed and she is able to fast.