Unforgivable Sirius

Challenge on Potions and Snitches by Romanticide: Sirius does something to Harry that he shouldnt and he shatters the poor boy's last hope of finally having a family. He's lost and hurt and terrfied with no one to turn to. No one but the man who has made his life miserable anyway he could. How can Snape help Harry,when he hates him himself; or does he? *Mild character bashing*