Must a girl re-fast the entirety of Ramadan if she gets her menstrual period in the last two days of it? And must she make up the days during which she doubted her menstruation when she was thirteen years old? And how can she perform the expiation if it becomes incumbent upon her and she is unable to feed the poor?

You are required to make up for the last two days of Ramadan (definition), during which you confirmed reaching puberty by the onset of menstruation. You are not required to make up for the days before that in Ramadan, and no attention should be paid to the doubt of menstruation starting before that until you are certain of its onset.

You have fulfilled your obligation if you made up for the two days during which you confirmed reaching puberty before the start of the following Ramadan.

If you delayed fasting these two days without a legitimate Shar'i (Islamic legal) excuse until the following Ramadan began, then you are required to make them up, along with giving a mudd (a measure) of food for each day to the poor. It suffices for you to give the food or its monetary equivalent to a family member or relative to deliver it to those deserving of it.