As a fellow Beauty God, that woman surpasses her in status, fame, and the power of her clan, even casting her aside to enjoy the world's highest praises alone. This makes Ishtar more envious of Freyja than anyone else.
A woman's jealousy is a capricious, rude, and terrifying thing.
It makes them lose their reason, lose their sanity, descend into malevolence, and turn the world upside down regardless of the consequences.
Such is the fearsome nature of a woman's jealousy, let alone that of a goddess.
As omniscient and omnipotent beings, their jealousy can cause a person's fate to spiral out of control, bringing chaos to the lower realms again and again.
Now, Ishtar is in such a state, willing to cooperate with the remnants of the dark factions to defeat Freyja.
Relying solely on her clan's power would never be enough to match the Freyja Clan.