How we cite our quotes: (Part.Chapter.Paragraph)
Quote #1
She had lost her position as first wife, and then she had heard the sounds from the bridal tent—laughter and muffled cries of pleasure. (1.2.51)
Not only does Rachel lose her position in the family, but she was also duped by Zilpah about what sex is like. Naturally, this precipitates a nasty form of jealousy that never really disappears.
Quote #2
Rachel was furious when she learned that Leah was with child. (1.2.58)
So, to go along with Rachel's jealousy regarding Leah's marriage, the resulting child then ticks her off even more. It's sister-rivalry in the making.
Quote #3
I suppose it only natural to assume that Leah was always jealous of Rachel. And it was true that Leah did not sing or smile much during Jacob's week with Rachel. (1.2.61)
Though Rachel was always jealous of Leah for marrying Jacob first and bearing him his first child, Leah was always jealous of Rachel for being blessed with great beauty. See, it goes both ways. But we can't be too surprised: as Bronze-Age women, all Leah and Rachel have is their marriageability.