Below is the first look of Sakshi as Ganga.
Those who don't know below is a gist about these three characters.
When the eight Vasus visited Vashishta's ashram with their wives, one of the wives took a fancy to Nandini and asked her husband Prabhasa, to steal it from Vashishta. Prabhasa the vasu,stole it with the help of the others, and was cursed by Vashishta to be born in the world of men. The seven Vasus who assist in stealing Nandini have their curse softened to be liberated from their human birth as soon as they are born, but Prabhasa, due to his being instrumental in the theft, is cursed to endure a longer life on the earth, though the curse is softened so that he becomes one of the most illustrious men of his times.
The youngest brother is born as Bhishma the youngest son of Shantanu by his first wife Ganga (the holy River), The other 7, were born as the older siblings of Bhishma, who were drowned by their mother Ganga as soon as they were born, thus fulfilling the softened curse on them.
Shantanu was the 12th king of a line starting from Dushyanta and Bharata, though the vansh (family or progeny) is said to have started with Bharata the great.
Bhishma learnt political science from Brihaspati, the guru of the Devas, Vedas and Vedangas from rishi Vasishta, and archery from Parashurama, also known as Bhargava, thus becoming an exceptionally skilled administrator, as well as an undefeatable warrior. His banner in battle was a golden palm tree.
He was known as 'Bhishma Pitamaha' (i.e., Bhishma, the grandfather or grandsire) among the Pandavas and the Kauravas. Bhishma is considered to be one of the greatest examples of a dutibound officer.
Shruti