I agree, I think Rosetta truly loves Maxi as her sister, but that love is laced with fear and guilt because of their father.
She tells her that there is nobody in the world who could understand her as she does, which is true in some way. She truly believes she knows exactly how broken Maxi is, because she was forced to watch everything the duke did to break her. In that way, she really is the closest thing Maxi will ever have to someone understanding her trauma, but it's not true that they cannot heal, her saying that just comes from a place of helplessness which she has lived in since she was 10.
She thinks there is no hope for either of them, because if she thinks it's impossible for her to love and trust, then it must be the same for Maxi who always had it worse. On some level, she sees Maxi's future as her own inevitable fate. If Maxi cannot trust, how could she ever hope to do so.
I think Maxi growing and healing will be good for both of them, and Rosetta will come to look up to her older sister with hope instead of defeat some day. At least I hope so.
I think people misunderstand that good characters need to be likable. Rosetta is a good character because she’s complex and flawed. The readers came in here, hell Maxi came in here, believing she was the golden child showered with affection. But that’s not the truth. Having to watch your own sister get beaten repeatedly and then being threatened that she could be like that — and BELIEVING that it might be because of you that your sister is being beaten so hard — made sure that Rosetta was never less than perfect. Her trauma was carrying guilt and fear. I don’t get why people are playing trauma olympics here and going “bUt MaXi hAd iT wOrSE!!!”, because Rosetta never said that.
She’s cold hearted and emotionless on the surface, but my personal take is that she was trying to bond in some way with Maxi through their shared trauma, on different sides of the glass. It’s interesting to see Rosetta, who’s so apparently calm and confident, be so resigned and almost helpless when it comes to trusting others. She was bonding with her big sister in that moment, as fucked up as it may be to trample on Maxi’s confidence by insisting that they would both never trust others fully.