Gifting (Hebanama)of undivided share in a muslim joint family property.

Gifting of undivided share in a muslim joint family property We have farm (Bagh) ancestral property co-owned by 6 other brothers. Couple of days earlier one co-owner gifted his share to his sister's son.without taking permission from other co-owners. My concerns are. 1. Can he donate/ gift the property without our knowledge? 2. It's an undivided property but through mutual settlement an shop was under his possession. So if the deed mentions the proper dimention of the shop. can the deed be considered null and void? 3. What action must be taken by us as it doesn't fulfill the delivery of possession condition of Heba. 4. Doner is around 90 years old and suffering from so many diseases.can we challenge heba on such ground. 5. As property is undivided.in heba he has mentioned the side of farm land.even it is not divided.can it be null n void. 6.co owner had done oral heba to his brother's son 6 months back.now co owner has done registered Heba to his sister's son.which one is valid and what action should take.