Can my mother gift her 50% of my deceased brother's property to 2 of 3 remaining children?

The property can easily be transferred by deed from your mother to the two children, but she needs to be appointed executor (if there was a will) or administrator (if no will) of your brother's estate, because an executor or administrator's deed is required to make the transfer. Until a legal representative is appointed nobody has any legal right to do anything with your brother's assets. Although your father doesn't need to be part of the transfer, he should be, to ensure there is only one deed, and that deed sets out all the ownership rights of everyone. Once your mother is appointed, she and your father should execute a new deed that gives 25% to each of the two children, and 50% to your father. They will also need to decide how they want to hold the property: as joint tenants with rights of survivorship (meaning when one dies, the other two automatically take half of the share of the deceased) or as tenants-in-common (meaning when one dies, that person's next of kin inherits their share of property).