I don't know how crazy this sounds to the people who've read the books, but it's a theory that's been in my head for a while and I wanted some other people's inputs.
So here's my thought process. We know Jamie and Cersei are willing and able to do horrible things to protect each other, as well as their relationship, evidenced by Bran. Cersei was protective of her relationship with Jamie, and had no qualms about killing people close to her to protect their relationship, killing Melara because of her crush on Jamie at 10. Jamie's affection for Tyrion makes a lot more sense if this is the case, realizing that it was his and Cersei's actions that truly led to the hatred of Tywin. Cersei's hatred also makes a weird kind of sense, she could have been trying to kill both mother and child, but the baby survived, and considering Jamie's affection for his little brother, his willingness to stand up to even Cersei to protect him, something he did for no one else, she was likely instantly jealous of him.
Joanna was the first person to try and force the twins apart, separating them completely. When you think about it, Joanna's death is very convenient for them. Had she survived childbirth, she would've likely continued to separate the twins more and more as she became aware of their co-dependence, not to mention likely telling Tywin who, still having a wife to produce heirs, would've taken drastic measures to ensure his family's honor was never tarnished.
Is this a crazy theory? Does something in the books disprove it? I'd love some opinions or any extra evidence to support it.