How many seconds? Depending on how the fight is taken(no talk, or talk in fight included), it could take up only part of the chapter, or most of it.
If it is the former, then she will get taken out quickly and start lashing out that she should be better because she has skillz... then MC will kindly, but bluntly tell her that effort can mitigate the distance or something of that scale.
If the latter, then he may remain silent, but have a happy and relaxed grin, showing that nothing has changed to her, or she will continuously yell at him, saying he WILL lose, regardless of his silly dancing(or something like that), then he will kindly refute it by doing some talking like, I am always your big brother, and they are meant to protect and teach their siblings.
Either way, she gets back her mindset that her brother truly is strong.