Is breastfeeding after the age of two—specifically at the age of ten—valid, and do the daughters of the maternal aunts become sisters to the suckling, and what are the implications of that, especially since they interact with her as mahrams?

The prohibitory suckling is that which occurs within the first two years of life and consists of five or more separate acts of suckling. Breastfeeding a ten-year-old girl does not create a prohibitory relationship, so her cousins remain non-mahram (marriageable). One must be careful to avoid forbidden mixing, watching movies, and listening to music.