Everyone is freaking out way too much. It's weird yes but she has no frame of reference for what's happening. Suddenly a presumably hot guy is in the place of her aging grandfather. It's supposed to be played for laughs. Y'all don't read enough manga if this level of humor is freaking you all out. She isn't really going to be banging pappy.
She's got a cute and humorous impression. Her mouth and clingyness is similar to a cat. Likely intentional.
I wanna see how this goes and grandma reacting. You can imagine the granddaughter saying "I see why you fell for him" or inquiring about pastimes as ways to get a date.
She'll likely be the occasional comic-relief rather than a focus and was introduced early as a means to get the couple into modern settings. E.g. Single throwaway panels saying "granddaughter recommended/suggested this..."