So what's he supposed to do? Walk into her room and dick her down into submission like "real man"? Seems like you just don't understand the premise of the manga and continue reading it. The title is: "My Friend's Sister Is Scary but Nice" if he wasn't supposed to be scared of/intimidated by her, then the title would be "My Friend's Sister Is Nice" and we'd be reading a different manga.
this is the right question, tho. She isn't scary. At all. She's quite nice, in fact. This is a fundamental problem with the writing.
She's not scary enough to warrant the over the top reactions she gets from him, so he comes off as a coward loser, not as a normal-but-slightly-shy guy his personality is supposed to be. That's my takeaway.
I like this, it's good, short and cute. But the characterization is lacking imo. He is way too jumpy and way too scared of her for no acceptable reason in universe. This has an easy fix, tbh, but as of now ch. 17 this is how i feel about the MC.
Now that i think about it, there was a manga where the MMC was scared of the FMC but it made more sense because the MMC was characterized as particularly cowardly and meek and the FMC looked like a ghost and was a little crazy. It was more comedic in tone than this, but that's what i'm trying to get at with the "right characterization"