What I think is maybe Bihaan fell off the cliff and was in coma for a few years looked after by Aryaans's family, Aryaan was similar to Bani's age then..Then this Aryaan met with an accident when he grew up...His face got damaged and doctors gave up all hopes for Bihaan... The doctor transplanted Bihaan's face to Aryaan.. Then Bihaan died but Thapki's belief stayed alive as Bihaan came back to her as her son-in-law.. May be after knowing the truth and by some means like kosi etc , Thapki too dies soon...So that we may rejoice Niyaan's scenes happily 😃