I gave my boy a sippy cup from 4 months and he got the hang of it reasonably quickly.  I think you could totally start now, especially seeing she's formula fed  They don't tend to drink huugge volumes anyway.
  They don't tend to drink huugge volumes anyway.
 
The best one I found was the Nuby with a soft spout and handles.  Can get from baby city and the warehouse I think.