Is it permissible to warn two girls against their friend who slanders them and impugns their honor, especially since there is an intention to propose to one of them for the brother, and the family is hesitant due to prior information about these slanders?

You must prevent your friend from slandering the two sisters and defend them, for the Prophet, peace and blessings be upon him, said: "Whoever defends the honor of his brother, Allah will keep the Fire away from his face on the Day of Resurrection." And his saying: "No man deserts a Muslim man in a place where his sanctity is violated and his honor is belittled, but Allah will desert him in a place where he loves His help."

You should also advise the two sisters to be wary of this friend and to regularly recite the adhkar (remembrances).

As for your spreading your friend's slanders and informing your family about them, this is not permissible, and you must repent to Allah for that.

If your family's intention is to propose to one of the sisters for your brother, then you must perform Istikhara (prayer for guidance), and do not let the words of the slanderer deter you.