Okay both stories are a bit messy, and I think the mails so far haven't given full information.
Sahil-Ganga-Tripti
They were happily married, then Ganga joined business and Sahil started to feel insecure because she came up with ideas that were appreciated more than his. In the mean time, Ganga started suspecting that Sahil was having an affair with Mohini (Harsh's wife) and even that Mohini was pregnant by him. So she in her depression and anger started treating him really badly at business. It turned out that the Mohini affair was a complete myth, Mohini had made Ganga suspicious for her own nefarious purposes. But because Sahil was now being treated very badly by Ganga, and couldn't understand her changed behaviour, he was depressed and got close to his secretary Tripti. Then when Ganga found out about this, she thought Sahil was serially unfaithful to her and wanted to divorce Sahil. It turned out Tripti had also been set-up by Mohini to trap Sahil. So finally Ganga left Sahil and he married Tripti. After the divorce Sahil refused to let Ganga come near the children, so she went away and became a successful businesswoman. She came back to meet her children and to take revenge on Sahil. Right now she is more concerned about getting her kids love than on the revenge part. She keeps visiting SN because she knows everyone there. Anyway, Sahil has a pretty miserable marriage with Tripti since she is only after money, and he is regretting leaving Ganga, but Ganga still hates him.
Karan-Nandini-Tanya:
Even more messy. Karan and Nandini loved each other but through circumstances Nandini got married to Ansh, who was Tulsi's son but was brought up by Aditya Gujral and was out-and-out evil. Ansh married Nandini only because he wanted to take revenge on her and treated her pretty badly. Nandini turned to Karan for help, and Tulsi was manipulated by Ansh into believing that Karan and Nandini were having an affair and she threw Karan out of the house. Initially Tanya was Ansh's friend who was helping him to separate Karan and Nandini, but at some point she turned positive and during the Ansh-Nandini marriage, she even gave Nandini shelter in her house when she ran away from Ansh. During this time, Tanya also started developing feelings for Karan, but they were one-sided and she didn't take them any further. Bottomline, after a lot of drama, Ansh raped Nandini, she filed a case against him and lost, got divorced from him, and Tulsi realized the truth and promised to get Karan-Nandini married. Tulsi also killed Ansh. Anyway, KN got married but while giving birth to Bhoomi (Ansh's child from the rape) Nandini slipped into a coma. At this point Karan was shattered and started hating Bhoomi, so Tanya was taking care of Bhoomi. Then the family decided to get Karan and Tanya married. Karan initially ignored Tanya, but then he started having feelings for her and they slept together. Then Nandini got well, and the doctor said she was not to recieve any shock. So everyone hid K-T marriage from her. Tanya got pregnant and decided to leave because it would be bad for K-N marriage. However, she was still attached to Bhoomi and would come and visit her secretly. Karan was more or less stuck between the two women. At some point Nandini came to know of the marriage and that Tanya was pregnant. At about this time, Aditya Gujral (Ansh's adoptive father) came back and he wanted revenge for Ansh's death. He also wanted to take Bhoomi away, so in desperation Nandini killed him. When she went to jail for it, she reunited K-T. After this, K-T were shown having normal relationship. But after the leap, it seemed K-T relationship had become bad because Nandini had gone off from jail without telling anyone and Karan could not forget her. When Nandini came back, she again told Karan to return to Tanya rather than think about her, and it seemed that Karan finally understood. However, after long neglect from Karan, now Tanya has become suspicious that K-N are having an affair behind her back. So the situation is as volatile as ever.
Just an average relationship in the Virani khandan. Have fun. 😉