I don't have an authoritative answer to this. But I've read enough posts here on PC, involving situations where a therapist was seeing more than one member of a family, to feel as though this is not a good idea at all. Can it work? Sure... I suppose it's possible. But there are so many potential complications, none of which can be foreseen ahead of time, I personally just think it's a situation that is best avoided from the get-go, so to speak.