Mothers property

I think the flat was transferred by your mother in your brother's name.  (If this is not the fact, then clarify what is the correct fact). 

If what I understood is the fact, then you have no right whatsoever in the property.  Your mother need not have consulted you or anybody to give her property to your brother.  Now as the property is in the name of your brother and he has died, only his wife and her children are entitled to the same and nobody else.  Your sister-in-law is legally right in refusing to give anything to you from the said property.