Dumbledore knew Sirius was innocent, he is the one who engineered his escape by hinting at Hermione on how to do it. The most likely answer to this question is that Dumbledore told Snape.
Sirius was not proved innocent until after he was killed during the Battle at the Ministry during the summer between Harry's fifth and sixth years. It was proven because the ministry admitted that Harry has not been lying, as well as the fact that Sirius's Will was found.
By his own admission, Lupin believed Sirius was guilty of dooming Harry's parents and committing the mass-murder he was imprisoned for. He believed this throughout Prisoner of Azkaban, only understanding he was innocent when he saw Peter Pettigrew on the Marauder's Map.
He knew. It's how the Order got to the Ministry so damn fast, and in force. Snape pretended Harry was spouting rubbish so he could escape to somewhere Umbridge was not and contact the Order. He'd heard those nicknames often enough growing up to know exactly who Padfoot was, and what Harry was trying to tell him.
No one knew about Snape's true identity as a double agent but Dumbledore. Dumbledore didn't make the deal with Snape for him to kill him until some point in the Half Blood Prince, and therefore no one in the Order of the Phoenix know.
In 2011, Rickman said in an interview with the Los Angeles Times that he knew that Snape was a "double agent... and a very good one at that" from the start, despite the fact that Rowling had only written three of the books at the time "Sorcerer's Stone," the first film in the series was released.
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.
Snape pretends to not understand Harry and carries on like nothing happened. We know, however, that he would understand what Padfoot meant and, being the clever guy he is, he could deduce that Harry believed Sirius was being held captive at the ministry.
Because to a pureblood fanatic like Bellatrix, Harry is a Mudblood because one of his parents (Lily Evans) was a Muggleborn. Anyone with Muggle ancestry is one, including Snape since his father was a Muggle (Tobias).
Severus Snape is one of the most divisive characters in the entire "Harry Potter" series. He was a double agent, so many character details were kept a secret. His mother was a witch and his father was a Muggle, leading to the moniker "The Half-Blood Prince."
Crouch's approach was unethical but popular at the time. The reason given for not using Veritaserum was a belief Sirius was too powerful a dark wizard he'd manipulate his responses so it'd be ineffective.
It turns out (naturally) that Sirius was framed, and even after their twelve-year separation he and Lupin remain fiercely devoted to each other. By book 5, the two of them are living together in secret.
When James learned what Sirius did, he stopped Snape before he reached the Shrieking Shack, saving his life. Snape, however, had seen Lupin in werewolf form and was sworn to secrecy by Dumbledore.
And Sirius died protecting his godson. It is disheartening that Harry had to endure the pain, the loss, and Harry being Harry, had to bear the guilt of Sirius' death. Molly and the Weasleys indeed cared for him like a family. But Sirius was a connection to his past, a bond that brought him closer to his parents.
The story of Sirius Black and Marlene McKinnon first began in their sixth year at Hogwarts. She was new and he was more than happy to make her feel welcome.
The final book, Deathly Hallows, explains this. Snape HAD BEEN a Death Eater. He later renounced his connections with that group, but no one besides Dumbledore knew for sure whether he had truly switched sides, or was just faking it. Dumbledore, alone, knew the truth.
"The Lestranges should be here," said Voldemort quietly. In short Lord Voldemort was the sperm donor for the Lestranges as Bella's husband Rodolphus Lestrange was firing blanks. Hence why they had no child so Lord Voldemort said he would father a child with Bellatrix to help out his best deatheater friends.
Draco's refusal to reveal Harry's identity to Bellatrix was not because he liked him. It was because he believed that Harry was the only chance they had at defeating Voldemort. At first, Draco admired and revered Voldemort until he threatened his family's safety.
She Had A Fling With Voldemort
Her relationship was later confirmed with Harry Potter and the Cursed Child. The play claimed that Bellatrix had been unfaithful to her husband and had slept with Lord Voldemort after she was freed from Azkaban.
The answer is that Sirius wanted to deflect attention from the real Secret-Keeper. He probably hoped Voldemort would go after him so he could steer attention away from Pettigrew.
Yet despite the danger of his mission, and despite being hated by most of the wizarding world, Snape held on to the one thing that kept him going: the safety of Lily's son. He even sent his Patronus to guide Harry Potter to the sword of Gryffindor (a known Horcrux-killer) in a nearby lake.
However, Snape demanded of Dumbledore that his love for Lily (his reason for switching sides) be kept a secret, especially from Harry, because of the deep animosity he felt towards Harry's father and his mixed feelings towards Harry (who reminded him of both his love for Lily Potter and her death, and his animosity ...
Draco Malfoy's mother Narcissa was cold, cunning and devoted to the Dark Lord. But she was also a mother, which meant she was willing to risk everything to make sure her son was safe. When Harry survived Voldemort's Killing Curse for the second time, Narcissa pretended he was dead so she could get to Draco.
Her sacrifice gave Harry the power of love, a protection spell sealed with the blood spilled by her death.
Neville's role in the prophecy in Order of the Phoenix is frequently referred to in a hypothetical scenario where Neville was the Chosen One rather than Harry. It is often suggested that Voldemort chose Harry over Neville because of Harry's half-blood status.