Which is better in the sight of Allah Almighty: performing Hajj for the third time and taking my sister to perform the obligatory pilgrimage at my expense, or paying the debts of my sister's insolvent husband, who was involved in an accident and supports a large family, using the costs of Hajj?

If your sister's husband is in debt and unable to repay it, it is better to pay off his debt with what you possess and postpone your sister's Hajj, because settling the debt is more important and beneficial for the couple than her Hajj. She is not obligated to perform Hajj until she is able to do so.