What is the ruling on seeing menstrual blood for a moment and then it stops, for a girl who is ten years old?

The minimum age for a woman to menstruate is a full nine years according to the majority of scholars. If blood is seen after this age, it is considered menstruation, provided it does not fall below its minimum duration. Jurists have differed regarding the minimum duration of menstruation: according to the Shafi'is and Hanbalis, it is one day and one night; according to the Hanafis, it is three days and nights; and according to the Malikis, it is a single flow with no time limit. The most valid opinion is that of the Malikis, which is also the choice of Shaykh al-Islam Ibn Taymiyyah, who held that the default for anything that comes out of the womb is that it is menstruation unless there is evidence to indicate it is istihada (non-menstrual bleeding). Therefore, what your sister saw is considered menstrual blood, and its rulings apply to it.