Yeah, her Bond Lines shoot up the lewdness meter like crazy towards the end there. My personal head-canon is she sees the MC as a surrogate for her big brother-crush since he/she's a composite of Shirou and Rin.
I-Illya-chan, you can't get mana from sucking on my fingers...?What should I suck then, Master?Eeh? Hish hat sho? (Is that so?)Illya at Max Bond is dangerous!!