Interesting. I'm sure the repro ivory was much older than at the time of "carving". However, while I believe that the tusk was older, I also believe it was freshly sliced into blanks.

Question: would ivory freshly sliced from an old tusk be brittle or must the blanks have been cut a hundred or so years ago to be brittle?

I have no answers but your sister's explanation is that the chip was not "carved" in the same time period as when the blanks were made. Good enough for me. Recent reproductions.