‘And Rebekah said to Jacob her son, “Look, I heard your father speak to Esau your brother, saying, ‘Bring me venison and make me savoury meat that I may eat, and bless you before Yahweh before my death.' Now therefore, my son, obey my voice just as I command you. Go now to the flock and fetch me from there two kids of the goats, and I will make them savoury meat for your father such as he loves. And you will bring it to your father that he may eat, so that he may bless you before his death.” '
Rebekah's plan is to replace Esau with Jacob, and she acts accordingly. Note the introduction of ‘before Yahweh'. Isaac had not said that, possibly because he knows Esau will not be impressed by it, but Rebekah knows that Jacob will be impressed by it (compare Genesis 27:20).
The subterfuge cannot be fully justified. Both Rebekah and Jacob should have trusted Yahweh to carry out His plans in His own way. But Jacob certainly feels that the firstborn's portion is his by right and probably felt that that included the blessing. As the blessing included lordship over the brothers he was probably right. He felt that he was about to be cheated. Rebekah also knew and felt the same. And Jacob was her favourite son. Thus they had at least partial justification and felt they were only doing what was right and preventing an injustice. They would both pay a heavy price in the future as a result of Jacob's ‘banishment'.
On the other hand no credit is reflected on Isaac and Esau. Esau certainly knew that he had sold leadership in the tribe to Jacob, and even if Isaac did not know (which is unlikely) he should not have shown such blatant favouritism. He knew that what he was about to do was epoch-making, and showed the arrogance of an old man who thinks that because of his age he can do whatever he wants. Everyone comes out of this badly. But the reader of that day would probably come down on the side of Jacob. At least he had a valid oath on his side and was supported by a birth prophecy.