It's an often repeated myth that only married women wore tiaras. Some noble families might have this tradition. But most royal families, the British included, didn't have this rule. Even in Princess Victoria, who never married, wore tiaras. And as was already mentioned, both Anne and Margret wore frequently wore tiaras before their marriage.
I somehow don't think that Eugenie will chanel her great-granny's look on her wedding day. Wearing a tiara on the forehead was quite 1920s, and I think she'll opt for a more contemporary look.
It can't be ruled out that she'll borrow a tiara from the Queen, but I don't think that she will. Neither Kelly nor Zara did, both wore one of Anne's tiaras. Sarah's tiara is really beautiful, and both girls are very close to their mother. So I'll be really surprised if she wears something other than Fergie's tiara.