Discipline my 9 year old daughter

My daughter is having a problem with discipline that i dont know what to do? She does everything opposite of wat she is told to do and likes to cry a lot. She is very talkative in school that causes problems in her class she can't pay attention to class work without talking. I have  2 year old son too and they fight a lot she compares herself too much with him as if thou she is 2 years too. What can i Do? Please help me someone.

[margypops]

You see what happens is that you have a child and she is the apple of your eye she gets all your attention, then a few years later a little sibling appears, gets all the attention babies get,quite naturally, your first born feels hurt, usurped, this is the time to make her feel as loved as she has always been and I am sure parents do what they can, and sometimes its a normal progression ...She is fighting with him because she feels some jealousy, I suggest you have one to one time with her when he is doing something else, get Dad involved with playing, games , outside activities , Fun , reading board games  and walk in her shoes .Good Luck