It's all in the subtext and context. Like, right before we hear about the brother, we see Nik thinking that "brotherly feelings" don't sound quite right. After that, it's just in the way they frame how controlling/possessive the brother was, and how they frame him reaching out to her, it just...really gave that vibe to me and to others who read it. But even if we're reading it wrong and it wasn't SEXUAL abuse, there was definitely emotional abuse and control at the very least. It looked like he was isolating her from potential friends and hobbies, and persisted in stalking her when she was trying to get away from him. Whatever it was, it was bad enough that when she saw him after thinking she'd gotten away, she ran in such a blind panic to get away from him that she got hit by a car. So, regardless of whether it was sexual/physical/emotional in nature or a mix of all three, he did something that caused her genuine trauma :/
Also, the fact that they were blurbing out the pertinent bits seemed like it was too taboo for a shoujo manga to state outright, which really limits what it could have been...so that, coupled with the framing and context, leads a lot of us to believe it implied sexual abuse. Her reaction/thoughts are also quite typical for someone who has suffered from sexual abuse by a sibling/family member

Hmmm i'm getting a Leeeettle lost in this, there's to much planning and not enough explanation of any of the situation...I mean at first it was her avoiding romance flags which was funny and understandable but now we get her having huge weird over reactions to 'fate' and being a 'commoner and all that jazz and thrown in the not at all explained properly issue of 'trauma due to an older brother (which honestly....I REALLY don't get what was at all traumatic about the situations we were shown, I mean i understand the feeling of being overshadowed your whole life but either the manga didn't explain it properly yet or something was lost in translation cause like was he a sis-con or was she a bro-con or was there abuse? like nothing was actually explained in all those panels)
Ahhhh it's just kinda stagnating the past few chapters so i'm hoping for a bit more explanation coming up, also....the vomiting part was weird to me to BUT BUT i'm assuming she actually vomited as that was the moment she remembered exactly how she died? since it mentions that in the next chapter or so, but who knows