Parents are planning on splitting up. Is what my mother doing correct?

It is definately wrong. She is using your siblings as weapons against your father. Sounds like you know that it was both of them that wrecked the marriage, so thats good. If your father is the one not putting the constant pressure on them, in my opinion they would be better off with a little distance from your mother. What she is doing could cause permanent emotional damage to them. Divorces are never pretty, but they don't have to be an all out war. Good luck to you and help your siblings stay strong.