What is the number of days that must be made up from the fasting of the past Ramadan, if menstruation lasted for eight days, and only two or four of those days were made up?

If one doubts whether they did something or not, the default is that they did not do it. Therefore, the questioner's sister's default is that she did not fast on the two days she doubted, and she must fast six days to complete what is due from her.