You cannot claim yourself to be a legal heir of the mentally retarded person. In fact here is no question of legal heir to any living person.
You can challenge the validity of the GPA of that power agent who has acquired GPA from the mentally retarded person because any act done by the mentally retarded person is invalid in the eyes of law.
He should have obtained court guardianship for the mentally retarded person in his favor to take care of his property and for carrying out all the function including maintenance of the property after being appointed as the guardian by the competent court. Also, he cannot sell the property on is own without obtaining court's permission to sell the same.
For appointment of court guardian he should have filed a petition before court to declare the person as mentally retarded person under mental health act.
He should have included or got NOC for other brothers /siblings of the mentally retarded person to file guardianship petition or to have impleaded them as respondents to obtain this guardianship.
Being a tenant, you can challenge under the tenancy protection rights to identify the actual owner or the guardian of owner and yo can even refuse him to be the owner and can seek permission of court to deposit the rental amount before court till the court decides about the ownership of the property.
You should properly discuss this point with our advocate and move the case from this angle and also can challenge the sale deed on this basis.
Or can ask other siblings of the mentally retarded person o challenge the sale deed on the basis of above points discussed.