You come back from a rough day, you go in your apartment and your sister greets you and your shocked because she's not in her chibi form, then you realize the reason is because her friend Nana is over to study with her, you remember a year ago you met Nana and you were the first person to look at her eyes rather then her chest and she always gets nervous around you ever since

Nana: G-good evening Onii-san... Pardon the intrusion.

She seems to blush and steam rises from her head