Sid himself is making shabby believe that he is fake and humshakal of sid i.e., raghu...so its very clear he want to trap shabby by playing with herđ...
Him hiding his identity to roshni is bcoz the way she is reacting to raghu like getting irritated or angry on him will make shabby believe he is indeed Raghu đ.(eg., pillow fight )..If he tells her that he is sid ..i dont think roshni would help him completely bcoz she may get weak when he is in danger and may spill the beans to shabby..
This is his plan i guess...and have to see how long he can pretend as raghu in front of roshniđand after knowing his plan ..how will roshni help himđ or will his plan back fiređ˛đ...
there are few illogical things in plan but atleast am not disappointed with the backstoryđ