Did we ever have definite proof Sena had a sister named Bora? This might be another Multiple Personality Disorder case—Sena is so traumatized she can't leave her safe space (her room) so Bora wants to get rid of everything that hurt her so Sena can finally leave.
I don't think we've ever seen Sena unless Bora was present and this is yet another character noting how similar they are. I guess they could just be twins, but with how many different characters are mentioning this plus the MC also showing interest in finding Sena, I'm not ruling this possibility out yet.