Literally no one would ever do this unless they were specifically asked to. What kind of expectation is this???
"Bro why did you come to your own home when you could have, like, booked a hotel somewhere else in the middle of the day and ignored your best friend's birthday?"
Considering the Sister's location, where she was at distant that she needed to take either the train or car in order to get home, it's not so outlandish as you think for some to opt to stay at a hotel for the night if they deemed it necessary. This is a bit exemplified as the sister explained that the assistant professor had driven her home. This is, unless I'm mistaken, the same assistant whose daughter the sister tutors, and as such is close enough acquaintances that I doubt they'd have minded letting the sister stay over for the night.
She could have had the day off, that's the point. She left the house so the two of them could have time alone when she could have stayed home all day and celebrated with Hina, which I'm sure she'd have loved to (hence him oweing her one).
Except, she already made plans with Hina to spend the day with her the week after, as Hina had mentioned earlier in the chapter. So it's not like the sister was going to miss out on anything when it came to spending time with Hina. All in all, she didn't really give the brother anything special that warrants him owing her one.
I love how people in these threads completely excuse the absolute failure on the little brother's part, like the fact he's been in love with this girl for ages yet somehow has not managed to confess his feelings once and keeps purposefully being obtuse with her from the moment this manga began, but the sister coming to her own home like usual is somehow cock blocking that deserves scorn. Lol. Lmao even.
Hey, I'm not claiming the brother is completely faultless. But let's be honest, it's already been established that the way he acts, as well as his inability to quickly step up and proclaim his love is a result of his sister's "raising" and "conditioning" him as that. Almost every aspect of the brother had been molded by the Sister to fit her demands, and among one those aspects is complete obedience. Notice how for the most part, it's Hina who takes initiate; now sure you could say that's him being spineless (and I would agree to an extent), but that's also due to the sister simply not giving him her permission. Most of his actions taken are all done with the mindset that they wouldn't bother and/or upset his sister or put him in her crosshair.
If the Sister was actually supportive and actually wanted the two of them to advance their relationship (while also not minding that she couldn't hold the reins of said relationship), I imagine she could've gave him a quick text (any time of the day) saying, "I'll be gone for a few hours today. Take care of Hina, and try not to be a wuss." That would've actually not only shown the sister giving her permission/blessing, but also gave the brother the go-ahead and probably motivation to initiate something with Hina, and any failure to do so would lie solely with him.
Look, I know it's not great writing either. I wish the author wouldn't constantly make these scenarios myself because it is frustrating to read. But speaking in-universe, the sister literally hasn't done anything bad here, she's just going about her life. Blame the author if anything.
Trust me, I actually don't hate the sister, since I can actually understand her character and see her motivation. She's actually the most interesting character in this manga. If anything I dislike Hina who gets jealous at the drop of a hat, like over why the brother was in cooking club... I mean he went out of his way to prepare something for your birthday, and then you get jealous over him being in the cooking club, thinking he's doing it to get close to a girl...
But I digress. I have nothing against the sister, but I certainly wouldn't say that she did him any noteworthy favors, besides letting him live with her (although the biggest reason for that was so she can have someone cook and clean for her....).