1. Well, the property is the self acquired property of your sister and her husband.
2. that being so their legal heirs like son.grandson, daughter ect does not acquire right , title and interest in it on the strength of birth or through law of succession.
3. In other words their grandchildren or daughter in law have no share in the property as even their son , since deceased, had no share in it.
4. They can not enforce the claim by legal means and if they make any disturbance in the enjoyment of the property then your sister and her husband can file case for injunction.
Nothing to worry and they may inform the local police about such pressures.