On that note these three are already calling each other by brother / sister terminology. Part of that is due to Z's wanting a "brother" and coming into the meeting with that term in his head. My son who had been using the term half sibling with regards to T up to this point had always also wanted a brother and his desire for one has trumped the desired terminology. At this point I am fine with it.
Generally I would want the terms brother / sister when kids are living with another but here the relationships are already very natural so I am letting them take the lead. We shall see what happens when T is added to the picture this morning.