Petyr "Littlefinger" Baelish is one of the most conniving characters on Game of Thrones. Coming from humble beginnings, he has weaseled his way up the ladder, through murder and all forms of deceit. Despite this, he has been able to gain meager trust with Sansa. Though Sansa knows not to trust him completely, she also still accepts his council and aid. However, there are some things Sansa doesn't know about Littlefinger that could make her dismiss him once and for all. But does Bran know everything Littlefinger has done on Game of Thrones, and if so, will he tell Sansa?
As the Three-Eyed raven, Bran seems to know just about everything that has happened in the past, present, and will happen in the future. When he reunited with Sansa, he unfortunately displayed his power by reminding her of her wedding night to Ramsey. Still, this proved just how much Bran knows, so it doesn't seem outlandish to guess he knows all of Littlefinger's secrets, and there are plenty.
The most notable to Sansa is the fact that her Aunt Lysa killed her husband, Jon Arryn, for Littlefinger. Though Lysa kind of revealed this right before Littlefinger killed her, Sansa was probably too distraught over almost dying herself, to really catch the meaning of what Lysa said. It was Littlefinger that not only convinced Lysa to kill her husband, who was the Hand of the King, but also then convinced Lysa to send a letter to her sister, Catelyn, claiming that it was the Lannisters who killed Jon.
It was this letter that convinced Ned to accept the job as Hand of the King, and why he and most of his family then made their way to King's Landing, where everything pretty much fell apart.
To add fuel to the fire, Littlefinger also pretty much secured Ned's execution when he betrayed him at King's Landing. Ned told Littlefinger everything he discovered about Joffrey not really being a Baratheon and how he planned to have Stannis put on the throne once Robert died. Littlefinger said he agreed with the plan, and went with Ned to stand against the crown, along with the City Watch and Ned's men.
However, Littlefinger, who controlled the City Watch, then had them turn against Ned's men, killing them, and put a knife to Ned's throat. Ned was then taken into custody and later executed.
Though Sansa's probably smart enough to know Littlefinger was certainly not innocent in his role in her father's execution, she may think he was just a bystander, when he really, he put the whole thing into motion. This reveal could lead to a serious rift in Littlefinger's plans and possibly even his death, a moment fans have been waiting for, for a long time.