You cant have your cake and eat it too. You took the deposit to hold the date. Then it got cancelled so you get the deposit, but you resheduled a free event (not the same as a paying gig) now the original wedding is back and you refuse to shoot it and still want to keep the deposit. Thats not fair. Essentially you want her to pay you to shoot your cousin's wedding.
I agree...the deposit is for the purpose of holding the date and to cover you on that day in case of a cancellation. IMO, you shouldn't keep the deposit and not hold the date.....I think you have 2 options, either shoot the wedding as initially intended (and I assume that you're under contract to do so) or see if she would accept a refund.