It’s not so pashut with the siblings. They sometimes have a multilevel kesher to the OTD sibling , and if the parent ostracizes or enforces separation with the OTD, the others can come to resent the parents. Sometimes they resent the OTD for the stress on the whole family. On the other hand sometimes there can be no calm atmosphere unless the rebellious kid moves out. I think it’s very important to connect individually with each sib and find out how they are thinking about the situation, then deal with each of them according to their needs, to give them chizuk in being empathetic and davening for their sibling, but also realizing the need to distance themselves from OTD behavior.
