Are the heirs obliged to pay money to the sister's husband from the inheritance, after the deceased father reassured him concerning a partner who defrauded them?

The guarantee is valid if its purpose is to guarantee against the transgression of the third partner. However, if the purpose is to guarantee the money absolutely, it is not valid because it is a trust (amanah).

Since the transgression did not occur until after your father's death, his heirs are not obliged to provide the guarantee, because a guarantee becomes void upon the death of the guarantor before the right becomes due.