Sir Isaac Newton had no full siblings. He did have one half brother and two half sisters. Their names were Benjamin Smith, Mary Smith and Hannah Smith Pilkington.
Sir Isaac Newton's father died when he was an infant. His mother subsequently left and remarried. She was gone for nearly seven years and left Isaac to be raised by her parents. Upon his stepfather's death, Isaac was reunited with his mother. She brought with her two girls and one boy that she had during her marriage to Barnabas Smith. This was a slightly unusual family situation, and it is believed that Sir Isaac Newton was deeply impacted by his mother's abandonment.