We have no confirmation the brother is actually dating his friend as of yet; in fact we don't even have clear signals that they have romantic feelings for one another outside of "well they're a boy and a girl and they're close; boys and girls can't be friends". We have uhhhhh
They hang out often
She hangs out at the brother's place 1-2 times a week (umm ok?)
If you're in college, that seems pretty normal, unless one feels like boys and girls can't be friends.
You would think we would get verbal or visual confirmation at some point if that was the case; we don't. We just get confirmation that they are good friends and after meeting her big bro became less gloomy and more outgoing. If you have an example in the manga where they explicitly say they are dating, they are holding hands, being intimate in any way beyond hanging out, whatever, please point me towards them because I do not remember them. Also if you check Chapter 59, before this one; "Isn't it scary to think that someone you care about might be changing little by little without you knowing?" If you're already interpreting it as Sasaki having feelings for her brother I guess that changes nothing, but that is a very general statement to be said about anyone you care about starting to possibly drift away, familial, romantic or otherwise. I've felt that way before about drifting apart from friends for various reasons. And at this point, via chapter 61, it's clear that Sasaki has feelings for the male lead, she just still has other things she is dealing with. It's no mistake that almost immediately after chapter 59 we see Sasaki blushing and being a lot more clearly interested. As people have said prior, the only person she ever felt comfortable around drifted away from her, and as such she felt lost. Do you remember covid? I remember being sent back home from college and graduating into rather than a big wide world an isolated space where my friends and acquaintances were no longer around me, and oftentimes weren't in contact with me. All of my support network vanished over the course of a few months. It took me years to even remotely recover back to some semblance of the comfort around others and openness I had prior. This is Sasaki's Covid moment. She already had no friends, and now she lost her one strong point of connection. She's a teenager and immature and she lashes out at the person who she feels has taken away the little time she now has to be with her sibling. Jealousy does not equal romantic love. Envy does not equal romantic love. It's pretty clear that the main conflict is the hostility towards the friend because Sasaki has never properly expressed her feelings and her brother has tried to avoid the issue. Also, note that the friend does not come to their Christmas Party.
Ah this went on for too long
TL

R I'm not sure how you've misunderstood this for so long but I get the feeling you've been interpreting this wrong from the moment they mentioned that she seems happier around her brother. It's a benchmark of her growing feelings towards the mc, not a statement on her being a brocon; she usually doesn't even talk about him, jeez.