Along with Ms Sen, Mr Singh's brothers-in-law Veer Vikram Sen and Prithvi Vikram Sen also formally joined the BJP along with their supporters in the presence of the party's state in-charge Mangal Pandey.
Virbhadra Singh, a six-time chief minister of Himachal Pradesh, was recently announced as the Congress' chief ministerial candidate by Rahul Gandhi, who added that he was confident the party would retain power in the state.
In August, Mr Singh was involved in a power struggle, with some in Congress' top leadership pushed for the party's state chief Sukhvinder Singh Sukhu to be their candidate for the upcoming elections.
Jyoti Sen had unsuccessfully contested the 2012 assembly elections from Kasumpti as an independent.
He claimed that the BJP is all set to win more than 50 seats in the 68 seat assembly.
Assembly elections in Himachal Pradesh will be held in a single phase on November 9.