Wow! What a statement that just blew us away! So how would you acknowledge or answer that?
Hi, I'm another soonsor. I have a child whose uncle married her mother after the child's father died, as he was expected to do by local custom. She is not the only wife. Though strange to us, it seems to be common in certain regions under certain religions. I would think that "what a big family" would be an appropriate reacrion, or you could just not say anything at all! Your child can't do anything about it, and there are so many other things to talk about. It will make the family structure confusing for you though, as sometimes they will refer to the others as siblings, and sometimes they won't. :)
I might say something like "That's a lot of wives" and go on to ask him more about himself. Especially in a large family, maybe having someone particularly interested in him would be good. It would not materially affect the letters and I try to be careful not to undercut parents. They still have the authority. Even non-Christian, sinful parents (ALL of us are sinful) still have authority over their children and the children need to be subject to them. I don't know enough about the situations to risk undercutting that. If there is something that needs to be dealt with, I pray that the center will see it and intervene as necessary.