Hello there! sorry I'm late with my response... is there any documentation on the gift certificate that you can cite? I'm sure you have a receipt. You can always take her to small claims court to get the money back, but it's probably not worth the hassle.
If she's really a "pro" she wouldn't do this. I get the feeling that she might be a MWAC (Mom With a Camera) and it's "fun" hobby for her. I would never pull this crap, even if I had raised my prices. I survive based on my reputation for how I treat my clients. Word of mouth is everything so if she has a reputation for canceling, then she probably doesn't need the income and doesn't really care.
If she does shoot weddings, you can always give her a nasty portrait review on weddingwire.com if she's listed there. Otherwise, I'm not sure what other recourse you have.
I'm glad she reimbursed you! Hopefully you can find another photographer who would be more than happy to take some terrific photos for your brother and SIL!!