1)Does the will have any legal importance?
The Will as well as the bequest made in it are very much valid, in the event of anyone challenging it then you may approach the jurisdictional court of law seeking grant of probate of Will or else the Will may simply be acted upon and your mother can have a partition deed drafted as per Will in mutual agreement with your uncle i.e., the co-beneficiary of he Will, and get it registered for applying for mutation of the same.
2)Will the three daughters and the son get equal portions of the house?
The property can be shared as per the bequest made in the will, if the other daughters have been omitted then they may not be entitled for any share in the testamentary property.
3)All of the siblings were born after 1956 but before 2005. To take probhet of that will, does everyone of the heirs need to agree?
For seeking grant of probate, the other heirs of the deceased testator are to be impleaded as respondents to hear their objections, if any, for this their years of birth is not a criteria.
4)Can the son get the entire house, after my grandmother's death without giving any portion to the daughters?
The son either has to acquire the property as per the bequest made in the Will or has to share the property along with his sisters as per Hindu Succession Act.
5)Can my grandmother (alive now) legally give off her portion to anyone?
If the grandmother has been bestowed with only life interest in the Will then she has no share in the property, however if the Will is not being acted upon, i.e., if the Will is ignored and the family agrees for partition among themselves, then the grandmother is entitled to a legitimate share in the property which she can give away or relinquish the rights to that share in favor of anyone of her choice.
6) Can my grandmother make a new will?
If she is allotted with a share then she can execute a will to that particular share alone, and not for the entire property.