Yeah, this confession only made sense as long as Prem was angry w/ Simar and had no intention of taking her back. Now, since he had reconciled w/ her, they could have secretly just gone and married either way - either @ a temple, or @ a registrar's office, whichever they chose. Instead, they stupidly spilt out that Roli is already married to Prem, thereby royally screwing her over. Why would Sid want to marry her now?
Incidentally, I am finding this plot about Roli ridiculous. She's there for the month until she finds accommodation in her college hostel, and after that, she would move. Given that context, on what basis did Mataji, who's a total retard, ask that she marry Sid? First of all, the age relationships would be inverted - she'd go from being Simar's younger sis to older sis-in-law. But more seriously, how do you go from 'she plans to attend college' to 'we want her to marry now' overnight, and Roli's parents being just fine w/ it? And Roli's behavior in that house w/ Sid was totally inappropriate - as he told her, she was a stranger, and should know her limits. Even more stupid was Sid's overnight conversion - actually, he was not so much of a villain - everything he said & did was practical, and from an almost atheist pov.
About Simar herself - I dunno that selfish is how to describe her, but stupid certainly is. On one hand, she considers Roli the legitimate wife of Prem, in which case, what Pari and others told her was correct - that she was characterless and slept w/ a guy she wasn't married to. Otoh, she acts like she's genuinely Prem's wife, doing karva chauth & all that. Yet, she tells Prem that she wasn't married to him. Which is it?