You're a Parent, Severus (джен)

When Lily got home from her first year at Hogwarts, she found a new distance between herself and Petunia. As a result Severus and Lily became even closer to each other over that summer. The next year Lily found a book in the Hogwarts Library that describe an ancient ritual for creating a blood bond that would make two unrelated people kin. She convinces Severus to undergo the ritual with her and they become brother and sister. This means that when Lily dies there is another whose blood relationship with her is recognized by the blood ward Dumbledore places on Harry. To Severus Snape's chagrin he finds himself the last line of defense between Harry and the forces of the Dark Lord.