What do you call your first cousin's wife?

In the sense of what is the name of the relationship, that would be "second cousin-in-law", though if you are that close to your second cousin and his wife, you could just say, "cousin".

In the sense of how you would address her, it would be the same as with your second cousin. Just the name that she goes by. After all, "second cousin", or even "cousin", isn't a title like Uncle or Aunt is.