If you haven't yet booked your sister-in-law's ticket, BY FAR the easiest thing to do is just book it under her maiden name. That way, the passport and the ticket match, and that's the only thing any airline in the world will care about.

Why risk having some foreign security officer give you a hard time, when you can just book a ticket under the old name?

All other measures, such as carrying the marriage license, SHOULD work IN THEORY, but depending on where you're going, you never know....