This Sunday we have a story from Deivathin Kural(Voice of God) about Goddess Parvati & Lord Vishnu.
Lord Vishnu and Goddess Parvati are siblings. In Goddess Parasakthi's incarnations such as Dakshayini, Parvati, Meenakshi, Lord Vishnu doesn't seem to have been born as a sibling. But, during the incarnation of Lord Krishna, they seem to be siblings. That is, when Devaki gives birth to Lord Krishna, according to Lord Vishnu's order, Vasudeva goes to Gokula and gets the daughter of Yashoda and Nanda Gopa, who also gave birth at the same time as that of Lord Krishna. After he returns to Mathura, Kamsa arrives at jail and asks for the baby. The baby of Nanda Gopa is given to Kamsa, who tried to kill the baby. The baby kicked Kamsa, who fell down to the ground. The baby then went up in the sky and took the form of the universal mother. "Hey You Fool! I am not the one who had come to kill you". Thus, the Bhagavatha story continues...
But even before the Krishna Avatara, they were said to be siblings. Lord Vishnu and Goddess Parvati do not have any parents. Both are considered as Para Brahma Svarupam and responsible for worldly activities through their Maya(Illusion). Since they do similar activities, they are considered siblings.
Lord Muruga is referred as the nephew(மருமகன்) of Lord Vishnu since he is the son of Lord Vishnu's sister, Parvati. He is also referred as son-in-law(மருமகன்), as he married Valli and Devasena, who were considered as the daughters of Lord Vishnu.
