Is it permissible for my sister to break her fast in Ramadan due to two menstrual periods in one month, or should she suffice with breaking it for only one period? And is she required to make up those days or offer expiation?

The maximum period of menstruation is fifteen days, and the minimum period of purity between two menstruations is thirteen days. If the total of the two blood flows and the intervening period of purity does not exceed fifteen days, then it is considered one menstruation. She performs ghusl after the first blood flow and prays and fasts. If the blood returns, she reverts to being in a state of menstruation. However, if the total exceeds fifteen days, then if the period of purity between them reaches thirteen days or more, the blood is considered a second menstruation. Otherwise, it is considered istihadah (non-menstrual bleeding), and she should not abandon prayer and fasting. The days on which she broke her fast must be made up (qada'). There is no expiation (kaffarah) due on her unless she delays the making up of the fast until another Ramadan, in which case she must pay a fidyah (expiation) of feeding a poor person for each day, unless she was ignorant of the impermissibility of the delay.