Our child’s bio sibling had a baby. How do we figure this out?

We just learned that an older bio-brother (T) of our two adopted boys had a baby. T was adopted by a different family, and we all lost touch with him for a few years but just regained contact. The relationship with him “feels” like a nephew to us parents but of course they are brothers and call themselves that.  So now our sons figure they are suddenly uncles at ages 15 and 14.  Us parents can’t quite figure out whether to call ourselves grandparents or step-grandparents or great-uncle/aunt.  Any advice from others in a similar situation appreciated.  And in the meantime we’ll try actually asking T himself, we’re going to meet him and his new proto-family tomorrow!