Vishnu is not biological brother of Parvati. But still he is considered has her brother. When did he become his brother ? Is Vishnu also brother of Sati ?
Vishnu and Parvati are brothers and sisters in terms of tattvas, Same with Brahma and Lakshmi, and Rudra and Saraswati respectively.
Sri Brahmanda Purana says that Mata Tripura Sundari along with her husband Bhagawan Tryambaka Bhairava Shiva creates the whole universes along with Tridevs and Tridevis as follows.:
A. From the left eye, which was of the nature of Soma (moon) came Brahma and Lakshmi.
B. From the right eye, which was of the nature of Soorya (sun) came Vishnu and Parvati.
C. From the third eye, which was of the nature of Agni (fire), came Rudra and Sarasvati.
I hope this clarifies your queries. Prd..
In Srimad bhagavatham the following are the slokas in which Sri Krishna speaks with Devi or yoga maya
gaccha devi vrajaṁ bhadre gopa-gobhir alaṅkṛtam rohiṇī vasudevasya bhāryāste nanda-gokule anyāś ca kaṁsa-saṁvignā vivareṣu vasanti hi -- SB 10.2.7
'O Devî, so auspicious, go to Vraja beautiful with her cowherds and cows, where Rohinî and other wives of Vasudeva are living in the cowherd-community [Gokula] of Nanda in seclusion out of fear for Kamsa. In the womb of Devakî there is the embryo known as [Ananta-] S'esha that is a plenary expansion of Me; make a smooth transfer by motivating Him to move out of her into the womb of Rohinî.
Then will I with My full potency do My share in becoming Devakî's son, o all-auspicious one, while you as well will appear as the daughter of Yas'odâ, the wife of Nanda.
The people will with incense worship you as the fulfillment of all their desires in different forms of sacrifice, because you for all that one may desire are the one capable of bestowing the blessings.
Depending on the place on earth [**] will you be given different names like Durgâ, Bhadrakâli, Vijayâ, Vaishnavî and Kumudâ, Candikâ, Krishnâ, Mâdhavî, Kanyakâ [or Kanyâ-kumârî], and Mâyâ, Nârâyanî, Îs'ânî, S'âradâ and also Ambikâ.
As they are both born or transferred between wombs of same mothers they became brother and sister. Also they are Narayana and Narayani.