What is the Islamic legal ruling on a second wife asking for the first wife to be divorced, and what is the Islamic legal advice directed to her?

The condition stipulated by the girl, that you divorce your wife, is an impermissible and legally void condition. It is not obligatory to fulfill it, due to the Prophet's (peace be upon him) prohibition against a woman stipulating the divorce of her sister.

The girl must fear Allah, because her demand is forbidden and involves destroying a family and harming a Muslim woman. She should not accept for another what she would not accept for herself, as the Prophet (peace be upon him) said: "None of you believes until he loves for his brother what he loves for himself."