Sadashivrao Bhau was son of Chimaji Appa and Rakhmabai and nephew of Peshwa Baji Rao I

Baji Rao I

Shrimant Baji Rao Balaji Bhatt , also known as Baji Rao I, was a noted general who served as Peshwa to the fourth Maratha Chhatrapati Shahu from 1719 until Baji Rao's death. He is also known as Thorale Baji Rao...

 and served as the commander of the Maratha

Maratha

The Maratha are an Indian caste, predominantly in the state of Maharashtra. The term Marāthā has three related usages: within the Marathi speaking region it describes the dominant Maratha caste; outside Maharashtra it can refer to the entire regional population of Marathi-speaking people;...

 army.
Sadashivrao, born in Saswad near Pune

Pune

Pune , is the eighth largest metropolis in India, the second largest in the state of Maharashtra after Mumbai, and the largest city in the Western Ghats. Once the centre of power of the Maratha Empire, it is situated 560 metres above sea level on the Deccan plateau at the confluence of the Mula ...

, was the son of Baji Rao's brother Chimaji Appa. His mother Rakhmabai died when he was barely a month old.His father died when he was 10.
He was cared by his grandmother Radhabai his aunt Kashibai. He was very bright from early
years.