It is believed that the deceased (your father) was Hindu, the property is self earned by deceased and deceased has not left any WILL, and succession has opened after year 2005 and provisions of THE HINDU SUCCESSION (AMENDMENT) ACT, 2005 shall apply.
You may go thru The Hindu Succession Act 1956: Sec 15
As per Hindu Succession Laws: The classes 1 legal heir is:
Spouse (your mother), 1 Brother, 3 sisters and they shall share the property equally amongst them.
The eldest sister thus has acquired share in property from her father, and her 1 son and 1 daughter shall inherit it.
Therefore the brother in law (husband of eldest sister) not being legal heir of your father shall not get any share.
It shall be appropriate to show all the documents and record to a local lawyer dealing in such matters, give inputs in person and proceed further as advised by your lawyer.