My brother's whole family died - property dispute

What do you mean by 'given to'? Was it through a registered deed?

In your earlier posting, you had stated a different version?

When did the fatal accident take place?

The other property was transferred to your brother by your father simultaneously, thus he became the absolute owner of the property transferred to him.

Now upon his intestate death, his class I legal heirs are entitled to succeed to the estates he left behind.

His legal heirs include his mother too besides his wife and children.

If his mother is predeceased and there is no class I legal heir surviving his death, then his father's legal heirs are entitled to succeed to the states left behind by him. You being one of the legal heirs of your deceaed father, you are entitled to succeed  the properties left behind by him (as per Hindu Succession Act), presuming that you are a Hindu by religion.

Your brother's wife's own relatives namely her brother or any other person from her parents side do not have any rights in the property because it was not her property neither she has acquired any rights in the property before her death. 

If they still agitate over this, you may file a suit for permanent injnction against them to restrain them from interfering in this property.

Yes.  Very well advised by Sri T.Kalaiselvan sir.  Nothing to add.

The brother in-law of deceased has neither any right, claim, nor any interest in  the property left behind by your brother.

Being class I heir of deceased the property shall devolve upon you.