If Charles refused to loan a tiara for one of them (which would be a bit petty if all others are wearing ones) one of the sisters could just buy one. There are some old tiaras for sale that aren't terribly expensive, and there are also tiaras made converted from necklaces (and maybe they already own those). And can be sold later again, the price of a tiara used in a coronation should not decrease at the very least. Many buy jewelry to be investments specifically. People don't buy tiaras anymore much but it used to be done.
I do not know if they would care enough but that's what I would do if everyone was wearing tiara in the family and there should be more excuses to buy jewelry
They are art imo.