Assalamualaikum. My sister give suckle to her 9 years old child. Is it permissible? What is the exact maximum age to breastfeed own children?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The mother’s breastfeeding her child after he completed two full years does not usually benefit him, especially if he has reached the age mentioned in the question. Besides, Allah, the Exalted, declared that the child’s completion of two years marks the end of the breastfeeding period. As such, after two years, children do not suffice with their mother’s milk, rather, they need other kinds of food that help them grow up. As for the Sharee'ah ruling on that, there is no harm if you breastfeed your child after reaching two years. As for the Saying of Allah, the Almighty, (which means), {… and his weaning is in two years} [The Quran, 31:14], it is for guidance, not for obligation. Also, children need breastfeeding very much before two years.

Allah knows best.