It is believed by the Shaiva Agamas that Shakti is the manifestation of Sadashiv. She then manifests in various forms and can don the avatars of Bhavani, Kali and Durga. To assist Lord Shiva, She dons the avatars of Lord Vishnu. Hence, Devi and Vishnu are inseparable and parts of one divine power. This is why they are considered to be brother and sister.
According to Shakta viewpoint, the MahaLaxmi or the Supreme Goddess gave rise to three pairs of divine siblings -
• Brahma and Laxmi (or Lakshmi)
• Vishnu and Uma
• Shiva and Saraswati
Also, the paani-grahan ceremony of Goddess Parvati was performed by Lord Vishnu. He also did Kanyadaan of Mahatriprisundari to Lord Mahakaleshwar. Moreover, it is believed that Parvati was born at the house of Nanda and was sacrificed in place of Krishna.