What is the ruling on combining two wives—one of whom is the sister of a friend of mine, and the other I married in revenge for what my friend did to me—given my confusion and fear of wronging either of them?

Exaggeration in love and hatred and attachment to people is not praiseworthy, while moderation and temperance are praised in Islamic law. Marriage is a solemn covenant, and the choice in it should be based on righteousness and compatibility, aiming for chastity and building a righteous family. If you are able to be just between your two wives and fulfill their rights, then keep them both. Otherwise, keep the one who is more righteous and suitable for you, divorce the other, and give her her due rights.