Rebekah wasn't speaking of Esau and Jacob regarding any justice others may be suggesting. Instead, Rebekah worried about the death of her old husband Isaac, and right after that her son Jacob, because Esau planned to immediately murder the younger Jacob (once their father had died).
Rebekah had been very concerned about the way Esau thought and lived, now that he’d married a Hittite woman.
Gen 26:34-35 (NIV) When Esau was forty years old, he married Judith daughter of Beeri the Hittite, and also Basemath daughter of Elon the Hittite. 35 They were a source of grief to Isaac and Rebekah.
Beyond that, now she had heard of Esau's plans to kill Jacob. He had become furious now for more than one reason. He’d already given Jacob his birthright for merely some red stew. That hadn’t bothered Esau at the time, but now he remembered, just after losing their father’s special blessing.
Gen 27:36-41 (NIV) 36 Esau said, “Isn’t he rightly named Jacob[a]? This is the second time he has taken advantage of me: He took my birthright, and now he’s taken my blessing!” Then he asked, “Haven’t you reserved any blessing for me?”
37 Isaac answered Esau, “I have made him lord over you and have made all his relatives his servants, and I have sustained him with grain and new wine. So what can I possibly do for you, my son?”
38 Esau said to his father, “Do you have only one blessing, my father? Bless me too, my father!” Then Esau wept aloud.
39 His father Isaac answered him,
“Your dwelling will be
away from the earth’s richness,
away from the dew of heaven above.
40 You will live by the sword
and you will serve your brother.
But when you grow restless,
you will throw his yoke
from off your neck.”
41 Esau held a grudge against Jacob because of the blessing his father had given him. He said to himself, “The days of mourning for my father are near; then I will kill my brother Jacob.”
As you note, Rebekah learned of those plans to murder in revenge immediately.
GEN 27:42-45 (NIV)
When Rebekah was told what her older son Esau had said, she sent for her younger son Jacob and said to him, “Your brother Esau is planning to avenge himself by killing you. Now then, my son, do what I say: Flee at once to my brother Laban in Harran. Stay with him for a while until your brother’s fury subsides. When your brother is no longer angry with you and forgets what you did to him, I’ll send word for you to come back from there. Why should I lose both of you in one day?”
Again, Rebekah didn't like the way Esau thought and lived (i.e. with that Hittite woman); now he even planned to murder his own twin. The next verse is:
Gen 27:46 (NIV) Then Rebekah said to Isaac, “I’m disgusted with living because of these Hittite women.
In summary, she wasn’t concerned about Esau. She worried about losing Isaac and right after that Jacob (due to Esau murdering him); thus she asked “Why should I lose both of you in one day?”
