Land dispute

"she and her father obtained a notary from my mother"! What do you mean by notary?

Anyhow, the property belongs to your mother and she is the absolute owner. She alone can deal with the property at her discretion. Spouse of the predeceased son is not even a legal heir to her.

Take stay order from civil court against sister in law so that she can't sell said land. 

You cannot do anything even if your sister in law is selling the property at the instance of your mother because your mother has consented to sell the property thorugh her daughter in law.

You cannot file any injunction suit also, if your mother is mentally challenged then you may have to first obtain a court decree declaring  that she is mentally ill hence any sale of property on her behalf is legally not valid after that you can apply for guardianship to become her guardian and file a suit for injunction to restrain your sister in law from selling the property. 

Since your mother is registered ower of land andshe can alon transfer or sell the property and no body can sell or transfer the Land on basis of notarized documents.