What's the Name of That Book??? discussion

This topic is about
Bloodtide
SOLVED: Children's/YA
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SOLVED. YA Dystopian siblings. She gets an arranged marriage with dictator & he ends up in the rebellion. She struggles between being loyal to her brother & her husband. Shapeshifters, human-animal hybrids, body modifications Read 2005-2000. Spoilers. [s]
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I don't remember really that much of it only that it really impacted me for a long time and after 15 years I would like to read it again. Even if it's relatively sad and a lot of abuse happens.
Plot
The book follows two storylines, two siblings a boy and a girl (I think they were actually twins). And at the start, they are regarded as high nobility (I'm not sure if they were royalty), and she gets an arranged marriage with the antagonist of the story who is at this point already a ruler (I'm not sure if the two siblings were the royalty of their "own" kingdom or that they were from high nobility of the Antagonists' kingdom). She falls high over heels before the actual marriage. But the antagonist is a tyrant and abusive, pretty much two-faced.
Plot with Spoiler
On the night of their wedding, the antagonist cuts her Achilles heels so she can't run away and is forced into a wheelchair with the excuse that she can't run away from him anymore (in a way that he wants her to stay close to him). The antagonist also plans out to attack her brother and he ends up in the slumps hurt badly where he is saved by a female human-pig hybrid (she sort of becomes his foster mom). He joins the rebellion to overthrown said antagonist together with other people from the slumps (human-hybrids and people alike)
After this, the sister is scared to get pregnant and is very lonely and feels caged. She encounters a young kitten which turns out to be a shapeshifter that can turn into a cat and a little bird. This girl becomes a little spy between her and her brother. The antagonist doesn't know that this is a shapeshifter just a regular cat. The shapeshifter age as fast as a cat does.
Some time passes over the books and the sister manipulates her husband and the rebellion several times, by giving wins to her husband or her brother depending on the mood.
At some point, the sister asks the shapeshifter to transform her into the form of the shapeshifter (as human) and seduces her own brother, and become pregnant with him. The antagonist is happy that she's finally pregnant (I'm not sure if I remember this correctly, but I think the shapeshifter smuggled in birth control pills priorly so that she didn't get pregnant.)
After she gave birth to a son the antagonist allows her to heal her legs (the Achilles tendon problem). She enters some scientific (or doctor's?) lab that lets her modify her body to her wishes. She has to enter some sort of glass tube with a liquid. She gets her legs fixed and becomes sterile. She also orders a clone made from her son, but one that's stronger and ages faster which becomes a tool to help her brother.
At the end of the book, the sisters' action comes to light. The shapeshifter helps the brother but betrays the sister. The sister kills the shapeshifter (now an old lady) in her cat form and the Antagonist doesn't understand why she kills the cat that's so dear to her. The clone kills the original son.
Sometime in the story, the foster mum is tortured and killed.
Spoiler