How can I play split screen multiplayer online in call of duty modern warfare 3?

yea, go to the main menu. Use the server if you are going to play online. Create a private match using the locator frame. As far as multi-player i believe you have to concur with the "terms of agreement" to establish a beach head?

Go into the private match lobby, connect your sister's controller and it should say press A to join or something like that and press A. Then invite your friends to the game. Your sister does NOT need to have an account it should sign her in as a guest with is pretty much your Gamertag(1)

If you go to the multiplayer screen, turn on your sister's controller it should say "press A to join or something along those lines. Word of warning though, your sister has to have an online account to be able to play.

before you go to a private match turn on the other control and press A when it tells you to and if she has an account she can play there if not she can sign in as guest and it will still let you play

Can anyone tell me what is the right answer for this question?

unfortunately my friend you can only do that offline :(