Yeah from what I understand she lashed out and was physically abusive to others in response to the abuse/neglect that she received until she started becoming suicidal. This fostered a misunderstanding with the brothers, especially the teal haired one.
Right?! Its an understandable defense mechanism on her part, and the brothers I think didnt bother finding out why she was like that. To think one of them was "smart" one quick questioning with the citrus maid (i cant remember if that was her name) and they could have found out the truth Σ( ￣□￣||)
The second brother may be easier to understand because it looks like he received most of the brunt of his sister's outbursts. The "smart" older brother isn't exactly meant to be liked, just an interesting character. The MC describes him multiple times as a snake. He will cut off anyone, even his own blood, even people he may genuinely like, if it is harmful to him as the future successor of the family. So I don't think he bothered questioning too much as associating with his sister would hurt him in every way with the way that she acted. He didn't know because he didn't bother with something that would hurt him and his position as she noted at some point.
Its sad that to think how protective they are towards her but the real sienna would never know. You’d think he life would be different if she reached out but at the same time i understand in her situation its always hard to reach out with low self esteem and especially if you believe they hate you from the beginning due to being a child of a prostitute ┗( T﹏T )┛ poor child ╥﹏╥