Although they were stepbrothers, because Kunti took charge of them after the departure of Madri with her husband Maharaja Pandu, Nakula and Sahadeva were as good as the other three Pandavas, Yudhisthira, Bhima and Arjuna
SB Canto 3
Are the twin brothers who are protected by their brothers doing well? Just as the eye is always protected by the eyelid, they are protected by the sons of Pṛthā, who snatched back their rightful kingdom from the hands of their enemy Duryodhana, just as Garuḍa snatched nectar from the mouth of Indra, the thunderbolt carrier.
Indra, the King of heaven, carries a thunderbolt in his hand and is very strong, yet Garuḍa, the carrier of Lord Viṣṇu, was able to snatch nectar from his mouth. Similarly, Duryodhana was as strong as the King of heaven, and still the sons of Pṛthā, the Pāṇḍavas, were able to snatch away their kingdom from Duryodhana. Both Garuḍa and the Pārthas are pet devotees of the Lord, and thus it was possible for them to face such strong enemies.
Vidura's inquiry was about the youngest brothers of the Pāṇḍavas, namely Nakula and Sahadeva. These twin brothers were sons of Mādrī, the stepmother of the other Pāṇḍavas. But although they were stepbrothers, because Kuntī took charge of them after the departure of Mādrī with her husband Mahārāja Pāṇḍu, Nakula and Sahadeva were as good as the other three Pāṇḍavas, Yudhiṣṭhira, Bhīma and Arjuna. The five brothers are known in the world as regular brothers. The three elder Pāṇḍavas took care of the younger brothers, just as the eyelid takes care of the eye. Vidura was anxious to know whether, after winning back their own kingdom from the hands of Duryodhana, the younger brothers were still living happily under the care of the elder brothers.