Dhritarashtra was rejected for the rights of king and Marriage of Dhritarashtra, Pandu , Vidura

Dhritarashtra, Pandu and Vidura learned knowledge from uncle Bhishma . From birth time Dhritarastra didn't see but he had 100 elephant's power of his hands.
Pandu was the also strong but not much than Dhritarashtra . But he was kind hearted
Vidura was not strong but he was scholastic.
Vidura was the son of servant so Dhritarastra didn't tolerate of Vidura.
In Hastinapur culture only elder brother got throne of Hastinapur. So Dhritarashtra all times dream for being king in the Hastinapur.

Satyavati decided to marry of Dhritarashtra with Gandhari :

Dhritarashtra was the eldest son so Satyavati decided to marry him with Gandhari so she sent Bhishma to Gandhar .

The story of Gandhari :

Gandhari , the daughter of king Subala and the sister of Sakuni. She was born in Gandhar so her name was Gandhari. Beside that there were many names of her like subali , Subala digital etc.

Believing that in the past life Gandhari was the goddess of Knowledge. She afraid of dark. And her brother Sakuni loved much.

And she prayed of Lord Shiva and lord shiva gave blessing of hundreds son.

Gandhari married with Dhritarashtra :

After Bhishma's proposal king of Gandhar could not refused it because Bhishma came Gandhar with big soldiers. But Sakuni couldn't take it easily because Dhritarastra was blind and her sister afraid of dark.

But Gandhari accepted the proposal and married with Dhritarashtra.

Gandhari and Sakuni took oath :

After marriage Gandhari sacrificed her wish and covered her eyes with red cloth and being blind because her husband couldn't not see.

After seeing that Sakuni became angry and oath himself that he will destroy Hastinapur by his politics.

Dhritarashtra rejected for being king of Hastinapur :

Before announcing Dhritarashtra as king the names of ministry and lord of soldiers were announced. Vidura was announced for ministry and Pandu was announced for lord of soldiers. But when the name king will announce Vidura protested against Dhritarastra and told ' Dhritarastra can't see so in the thought of purana Dhritarastra can't being king' . So Dhritarashtra was rejected for being king and Pandu was announced for being king.

Pandu married with Kunti and Madri : 

One day kuntivoj organised kunti's marriage ceremony then Vidura and Bhishma Sent Pandu to marry with Kunti .

In this ceremony Pandu answered Kunti's questions and married with Kunti.

Some time later Pandu went to his friend Salya, king of Madra. And Pandu married with Salya's sister Madri.

The Story Of Kunti :

Originally Kunti was the daughter of Sur, father of Vasudev and grand father of Krishna and Balaram. At first her name was Pritha. Sur gave his daughter to Kuntivoj. Following of adopting father's name her name changed to Kunti. One day sage Durbasa came in Kuntivoj's Kingdom. Then Kunti nursed him a lot . Sage Durbasa was happy for her nursing. And gave her a mantra or shlok which could called any God and got son of the god.

In early in curiosity Kunti read the mantra of the name of Sun . So God Sun came there and gave her a son named Karna. But Kunti was unmarried so god sun gave her blessing for getting back virginity again and gave kavach and kundal to karna to protected him. So Kunti floated him into Ganga river and was adopted him by charioteer Adhirath and Radha. So karna was also called Radhee .

Vidura married with Sulabha :

According to the MAHASAMAR by Narendra Kohli, It is Paransavi (पारंसवी), she's the daughter of king Devak, from a Shudra woman. According to some other sources, it is Sulabha as stated in tale when Krishna arrives at Vidur's home while Vidur isn't present and his wife is bathing.

Dhritarashtra, Pandu , Vidura and theirs wife Gandhari, Kunti, Sulabha