There is one way for Roshni to come to her realization and go from being Ragini to Roshni is if something were to happen to Sid. As we saw how a small injury led to her admitting her feelings to him. Just thinking if something major happened to him, Roshni would forget everything and run to him. Either Sid would get seriously injured or some one from the sengupta family might treat him like dirt. If not Neel it will be the Sunny guy who will make Sid's life a living hell. If anything happens to Sid Roshni will forget about her promise to herself or the fact that she is engaged to Neel and profess her feelings.
SidNi are the same in the sense that if one is in danger or is injured the other forgets all other relationships and runs to the person. The recent office attack is a proper example. Sid had said previous to it that he would forget Roshni but when he noticed goons going to attack her office, the pati in him got worried for her and ran to her rescue. Then when he got hit on the head, the patni in Roshni awoke and yelling at her employees to go get help for Sid.
In this case, something has to seriously happen to Sid for Roshni to come to her senses and realize that what she is doing is also not helping those that she wants to protect.