Pretty sure her father raped her that's why she's acting like that.. I was pretty confused with page 21 and page 19. She must be a lot younger when her mother remarried and meet her brother
because that's manga for yah.
even if it doesn't have full ntr i wouldn't be a surprised if a guy comes into his sisters life and our MC get's really jealous.
Tho i don't think we should even worry about any near NTR because she said in this chapter.
"With 'siblings', it doesn't matter how badly they break, because we'll always be 'family'."
So i assume she's literally interested in her brother, not anyone else because of this.
Looking at how our MC is still immature and shallow minded, i smell some fucked up drama ahead, which force him to act like a matured person. Either someone would catch them doing sexy stuff or he got NTR'd. It's likely the latter.