Or HaChaim on Genesis 30:1/אור החיים על בראשית ל׳:א׳

1 ותרא רחל כי לא ילדה, Rachel saw that she had not borne children, etc. The Torah adds the word ליעקב, for Jacob, since it was clear that Jacob was not the one who was sterile. The cause therefore had to lie with her. When the Torah describes Rachel as jealous of her sister, this indicates that had Leah not been her sister Rachel would have attributed the fact that she did not conceive to genetic differences between her and another wife of Jacob. She could not account for such genetic differences when her sister was involved, i.e. someone who was genetically similarly equipped (she thought).

2The word ליעקב may have an additional meaning here. Her major concern was the fact that she was unable to bear children for an outstandingly righteous man such as Jacob. Had Jacob not been clearly righteous, she would have attributed his bad luck to a character defect in her husband. Knowing that she herself was righteous she could not understand why the promise contained in Psalms 125,3 that "the scepter of the wicked will not be allotted to the fate of the righteous," would not apply to their union. Had Jacob not been so clearly a pious man she could have attributed their lack of success to a shortcoming in Jacob. As it was this was impossible. Therefore, she was jealous of her sister who had succeeded where she had failed.