Why did Lysa poisoned Jon Arryn?

Littlefinger had persuaded Lysa to poison Jon’s wine with tears of Lys, suggesting to her that this would both remove the obstacle to their marriage and prevent Jon from sending Robert Arryn to Stannis Baratheon at Dragonstone. Lysa did as she was bidden.

Does Cersei know who killed Jon Arryn?

She tells as much to Jaime when Bran overhears them. Jaime says that Jon couldn’t have told King Robert before he died because Robert would have their heads on spikes by now. So Cersei knew that Jon knew.