Abusive behaviors and patterns are abusive no matter who is behind them, or what is causing them. Mental illness is a reason for abuse, but it is not an excuse for it. You do not deserve or have to tolerate or understand abuse from a mentally ill parent any more than you do from a non-mentally ill parent. Mental illness does not negate the harm and trauma that abuse cause.
As for whether one child can be more abused than the other, absolutely. This is a very common pattern often referred to as ‘scapegoat’ and ‘golden child’. The golden child is idolized by the parent and can do no wrong, whereas the scapegoat is blamed for everything that goes wrong in the parent’s life.