Man, the amount of gooner/groomer comments in here is staggering.
I mean, how many actual manga have you read to not recognize the touchy feely big sis archetype? It’s teasing because she sees him as a little (step?)brother that she’d never do anything with beyond flustering him.
Pretty sure the pay back later is taking care of her because she helped them out... or hell, maybe you could consider that grooming for favors. Maybe even a reconsideration as a partner when he gets older.
Today’s world feels out of wack, but it’s sadder that these considerations are necessary for the irl creeps…