Please advise whether a gift of share from undivided inherited property to a co-heir is valid under Muslim law.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

There is no harm in offering some of the inheritance to some heirs before dividing the inheritance or after dividing the inheritance if this is done with the agreement of the heirs, because it is their wealth and therefore they can do with it as they please. This is indeed legislated because offering something as a donation is something urged by Islamic Law which set a great reward for it. However, this is conditioned upon the donor being an adult.  

Allaah knows best.