1. After the demise of the husband of your sister, your sister and her son is the legal heirs of your demised brother in law's property,
2. Now get all the property of your demised BIL mutated in the joint names of your sister and her son, if already not done so far,
3. Change the electrict connection and tax bills in their names,
4. You sister can very well marry now and retain her share of the property of her demisesd husband along with that of her son legally,
5. She need not pay any heed to the threats or demands of her sister in law and agree to return any moveable and/or immoveable property of her demised husband to any body. There is no such law in our country by which widow shall have to return her demised husband's property to anybody, if she decides to remarry,
6. Record the conversation of her sister in law and lodge a police complaint immediately about her said threat and blackmail.