Add DCs to Remote Sites?

This is the situation.

I have 2 remote branches and the main branch. Currently I have only 1 DC at the main branch for all users? at all three locations to login. Because of slow logon times at the remote branches I want to put a DC and also use it as backup for our main server. BTW: our branches are connect via T1 P2P.

To do this do I need to create sites in AD on my main server?
What steps would be the best route in order to accomplish this.

I really appreciate all your help.

TIA