This morning I took my kids out to a local park wher they started a number of chalk drawings on a hand / paddle ball court. My daughter at one point while drawing indicated that two of the "girl" drawings were of her and her "Sister [T]". I had not prompted this at all.
More and more I have noticed they refer to T and Z as their sister and brother. I admit I have not done anything to stop or modify their terminology as I wanted their thinking to be organically derived. I do think their watching yesterday's Good Morning America segment where they saw Ryan Kramer and two of his half siblings refer to each other as brother / sister only confirms their belief that their use of brother and sister is OK.