It's just having a ally guild. Often comes about when there's more applications to join the guild than there are spots in the guild. However like any other ally, there's the risk relations eventually turn sour or cold. It takes work to keep a sister guild in good relations with the main guild AND make sure the sister guild is able to support its own members growth sufficiently. If a sister guild is there because there's not enough room in the main guild then it needs to be able to give the same kind of growth as the main guild or there's no point in being in a sister guild