Vyasadeva, following the order of his mother, Satyavati, begot three sons, two by the womb of Ambika and Ambalika, the two wives of his brother Vicitravirya, and the third by Vicitravirya's maidservant. These sons were Dhrtarastra, Pandu and Vidura

Expressions researched:
"After Vicitravirya's death, his mother, Satyavati, who was also the mother of Vyasadeva, asked Vyasadeva to beget children through the wives of Vicitravirya" |"Through the womb of the two wives and the maidservant of Vicitravirya, Vyasadeva begot Dhrtarastra, Pandu and Vidura"

SB Canto 9

SB 9.22 Summary:

The sons of Pratīpa were Devāpi, Śāntanu and Bāhlīka. When Devāpi retired to the forest, his younger brother Śāntanu became the king. Although Śāntanu, being younger, was not eligible to occupy the throne, he disregarded his elder brother. Consequently, there was no rainfall for twelve years. Following the advice of the brāhmaṇas, Śāntanu was ready to return the kingdom to Devāpi, but by the intrigue of Śāntanu's minister, Devāpi became unfit to be king. Therefore Śāntanu resumed charge of the kingdom, and rain fell properly during his regime. By mystic power, Devāpi still lives in the village known as Kalāpa-grāma. In this Kali-yuga, when the descendants of Soma known as the candra-vaṁśa (the lunar dynasty) die out, Devāpi, at the beginning of Satya-yuga, will reestablish the dynasty of the moon. The wife of Śāntanu named Gaṅgā gave birth to Bhīṣma, one of the twelve authorities. Two sons named Citrāṅgada and Vicitravīrya were also born from the womb of Satyavatī by the semen of Śāntanu, and Vyāsadeva was born from Satyavatī by the semen of Parāśara, Vyāsadeva instructed the history of the Bhāgavatam to his son Śukadeva. Through the womb of the two wives and the maidservant of Vicitravīrya, Vyāsadeva begot Dhṛtarāṣṭra, Pāṇḍu and Vidura.

SB 9.22.25, Translation and Purport:

Bādarāyaṇa, Śrī Vyāsadeva, following the order of his mother, Satyavatī, begot three sons, two by the womb of Ambikā and Ambālikā, the two wives of his brother Vicitravīrya, and the third by Vicitravīrya's maidservant. These sons were Dhṛtarāṣṭra, Pāṇḍu and Vidura.

Vicitravīrya died of tuberculosis, and his wives, Ambikā and Ambālikā, had no issue. Therefore, after Vicitravīrya's death, his mother, Satyavatī, who was also the mother of Vyāsadeva, asked Vyāsadeva to beget children through the wives of Vicitravīrya. In those days, the brother of the husband could beget children through the womb of his sister-in-law. This was known as devareṇa sutotpatti. If the husband was somehow unable to beget children, his brother could do so through the womb of his sister-in-law. This devareṇa sutotpatti and the sacrifices of aśvamedha and gomedha are forbidden in the age of Kali.

aśvamedhaṁ gavālambhaṁ

sannyāsaṁ pala-paitṛkam

devareṇa sutotpattiṁ

kalau pañca vivarjayet

(CC Adi 17.164)

"In this age of Kali, five acts are forbidden: the offering of a horse in sacrifice, the offering of a cow in sacrifice, the acceptance of the order of sannyāsa, the offering of oblations of flesh to the forefathers, and a man's begetting children in his brother's wife." (Brahma-vaivarta Purāṇa).