1) your sister is not liable to pay her late husband debts . her liability is limited to extent of property of her late husband inherited by her
2) refuse to pay . she should inform them that she has not inherited any property of her husband and is not in position to pay
3)if they create a nuisance lode police complaint of criminal intimidation under section 506 of IPC against collection agents
4) record their threats . it will come in handy for lodging police complaint
5) if possible install CCTV cameras around the house