SB 9.22.25

kṣetre ’prajasya vai bhrātur
 mātrokto bādarāyaṇaḥ
dhṛtarāṣṭraṁ ca pāṇḍuṁ ca
 viduraṁ cāpy ajījanat
Synonyms: 
kṣetre — in the wives and maidservant; aprajasya — of Vicitravīrya, who had no progeny; vai — indeed; bhrātuḥ — of the brother; mātrā uktaḥ — being ordered by the mother; bādarāyaṇaḥ — Vedavyāsa; dhṛtarāṣṭram — a son named Dhṛtarāṣṭra; ca — and; pāṇḍum — a son named Pāṇḍu; ca — also; viduram — a son named Vidura; ca — also; api — indeed; ajījanat — begot.
Translation: 
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.
Purport: 

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

“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).