As Purab mentioned earlier, Thapki and Sargam are both 22 years old. And we know Purab and Hansika are almost same age which makes Hansika older than Thapki. Then, Thapki's dad must had already been married or even had a kid by the time he strated dating Jaya. So, was Jaya a mistress? I don't think so. There is a high chance that Mukul lied about his marital status and made the poor girl believe he was single. It explains his sudden disappearance too.