The birthright carried with it the privilege of the special blessing from their father.

Isaac was old and his eyes were dim, so that he could not see; and he knew that the day of his death might be near.

(Genesis 27:1,2) He directed his son Esau to go into the field and take some venison and bring it to him that he might eat; and give Esau his blessing.
[111]The Bible does not clearly show that Isaac knew God had foreordained that this birthright should belong to Jacob, nor that he knew Esau had sold it to Jacob; therefore we can excuse the old father for thinking it was his duty to bestow his blessing upon his son Esau.
Rebekah the mother, however, knowing of both these reasons why Jacob was entitled to the birthright, knowing that Esau did not appreciate it, and knowing of his wrongful intent to deprive his brother Jacob of the privileges and blessings incident to the birthright, advised Jacob what to do in order to protect his own rightful and proper interests.

The mother therefore was carrying out God's will in this respect.

She was doing what all honest persons should do--try to protect the rights and privileges of those that are near and dear to them.