I don't have any great ideas but I DO want to say: Your husband is a very thoughtful person. We see so many posts on Mamapedia about dads/husbands who are clueless about gifts, hate to get anything for anyone, expect their wives to shop on their behalf, forget to mark milestones, etc. But you are married to a man who actually thinks about ways to show his nieces they are in his thoughts. Please give him a big hug today and tell him he's a pearl.
Your nieces also sound wonderfully appreciative, which many teens aren't. You are so blessed all around.
It sounds like your husband enjoys giving (and the girls like getting) nicely old-fashioned gifts. If he hasn't given bracelets before, consider something like a personalized bracelet for each girl with her name and the date engraved on a thin bangle, or a bracelet with a single charm that is meaningful for that girl (representing one's beloved sport, or the musical instrument that another plays, etc.). I am not at all a fan of the Pandora types of bracelets (there are similar brands) because they are very chunky and heavy once you have more than a charm or two on them, yet don't look decent with just one or two charms; the girls might not really wear them much if they don't want a heavy bracelet.
An "Alex and Ani" type of thin bracelet with much lighter charms and only one or two very specificially chosen charms for each girl would be easier to wear. You could include a gift certificate worth about the value of a few additional charms if you like. Be sure that your husband writes a note for each girl about why he (and you) picked the particular charm you put on the bracelet. These kinds of bracelets look fine with just one or two charms on them and don't look like they need to be "filled up" like the old link charm bracelets or the Pandora ones do.
But whatever you get them -- both your husband and these young ladies are worth appreciating!