I think that Tanya, is in fact feeling a little bit jealous (which I assume is going to escalate) because, she may have felt that there has been a turning point in teh relationship between Raj and her where Raj may have begun to accept her as a wife not just in name but in heart too. [Enhanced by the increasing no. of TAAJ moments)
Yes she knows about the past feelings about Raj and Trish, but she may have felt that as both of them are now married (ie Trishna\Rajdeep & Tanya\Raj), they would be fully committed to their respective partners, and therefore seeing Trish\Raj together she is starting to doubt her beliefs and her thoughts as to the actual status of her relationship with Raj.
I think that a return of Kanhiya would be a concept to explore to bring some drama into the mix. It would also serve as a medium of bringing Tanya and Raj closer together.
Another concept that can be introduced is probably bringing in Raj's real family and explaining why it is Raj is not close to them or maybe why they have not spoken to Raj. Bringing in a blemish to Raj's character, who seems so saintlike might also be a nice twist to the story.
(I'm not sure if this was ever explored in the serial before, but where are they?)