Mahabharat - Birth Story Of Dhritarashtra, Pandu And Vidura

After death of Vichitravirya the whole kuru family disappointed for thinking about their next generation because the wifes of Vichitravirya had no son when Vichitravirya was died.

Bhishma adviced the concept of 'niyog' :

When Vichitravirya was died , for next generation Satyavati requested to Bhishma that he sex with Ambika And Ambalika and gave birth theirs son. Bhishma told Satyavati that it was not possible but he gave advice to Satyavati the concept of 'Niyog' . 

What was 'Niyog' :

Satyavati asked what is 'Niyog'? Bhishma told Satyavati ' in purana if husband died but he has no son then high quality sage or gods can sex with the died husband's wife for giving birth son and protect husband's  generation . In this condition the new born son is called of died husband's son and protects his family.

Satyavati revealed her secret before marriage and told about her son ' Veda Vasa '

Knowing about ' Niyog ' Satyavati told her relationship before marriage and she had a son named sage Veda Vyasa. Bhishma told that it was good news and called him for niyog.

Birth of Dhritarastra , Pandu and Vidura :

For calling from his mother sage Veda Vyasa came in Hastinapur and accepted the proposal of Niyog and sex with Ambika , Ambalika and a lady servant. As a result Ambika gave birth Dhritarastra , Ambalika gave birth Pandu , and a lady servant gave birth gave birth Vidura.

Dhritarastra didn't see from his birth time but he had 100 elephant's power. So his name was Dhitarastra and he was the eldest brother.

Pandu was also strong and kind hearted. 

Vidura was very scholastic but he was servant's son Dhitarastra didn't love him.

Sanjay and Dhitarastra