Un registered sale deed worth of less than 100 rupees

Sir

My father purchased house plot of 500 yards in the year 1966 in andhrapradesh for Rs 35/- only and the document is not registered. In the year 2000 my father expired. We are three brothers and my mother is alive.

Now my query is

1. Whether the un registered deed is valid or not

2. how to transfer/register  the property in my mothers  name and how to register the same among the brothers