What is the ruling on borrowing money from family to cover the expenses of Hajj, on condition that the money is repaid before going for Hajj?

There is no harm in borrowing money as a Qard Hasan (definition) (a benevolent loan without interest) to cover the expenses of Hajj, whether it is repaid before or after Hajj. The Qard Hasan (definition) is permissible (mashru') and is an act of worship for the lender if they intend it as an act of kindness, and it is permissible (mubah (definition)) for the borrower if they know they can fulfill the repayment. As for borrowing with interest, it is Riba (usury) which is forbidden by the Qur'an, Sunnah (definition), and scholarly consensus.