I just take it as she still hasn't gotten over the shock of seeing her grandparents suddenly being only ~10 years older than her. So she just sees a hot guy that looks nothing like her grandfather whatsoever to her eyes and wants to... Y'know...
This plays into an actual thing, the Westermarck effect. And it has... bizarre... effects on people. There have been cases of older men and younger women meeting each other and being utterly fascinated with one another... only to find out that they're father and daughter. The intense attraction is basically seeing your genetic similarities even if you don't realize it and because you don't know it and weren't around each other to be conditioned against incest, you have a reverse reaction - you're attracted to one another. There was actually a documentary I watched where an older divorced/widowed (can't remember) guy met a younger woman online and met up and experienced just this. I forget how they found out, but eventually they did get a DNA test only to learn the younger woman was his biological daughter. And because they both wanted to reconnect with each other after learning that they were long-lost parent/sibling they decided to "inoculate" themselves against this incestuous desire by constantly visiting one another and doing everyday mundane tasks and, hopefully, not fucking one another. And it does work, given enough time the brain will recondition itself and you'll start seeing them as "family - do not touch" instead of "HOT DAMN! FOR SOME REASON YOU'RE JUST MY TYPE, LETS BANG!". Same way it would have worked if they were together as father/daughter from the get go, albeit not nearly as effective.