Alpha Gamma Delta uses sister-mothers/sister-daughters. Though I don't speak for my entire sorority, I know that the correct term (according to my revamped IHQ website ) is sister-mother/daughter:

In addition, each new member is paired with a sister-mother—an initiated member who serves as a personal guide and friend throughout your daughter’s first months in Alpha Gamma Delta.

Our new members find out who their sister-mothers are right after New Member Service.