Cersei sees her and Jaime as one person in two bodies. She has said so on occasion, the first time was in aGoT, when Ned confronted her about the incest. It is impossible for her to even think that Jaime could be the one who kills her in the end. He is her other half, their hearts beat as one, and she believes that they'll die together, since they came into the world together.
On the other hand, Cersei despises Tyrion, blaming him for Joanna's death, and doubts his every action. She cannot believe he'll do something to aid her, like how she is convinced he isn't looking out for Joffrey's safety during the battle of the Blackwater, whilst Tyrion is doing everything he can to keep Joffrey on his throne. Cersei cannot believe that Tyrion would do anything in her benefit. In addition, Tyrion is a small, little man. Cersei has already proven that she isn't the most cleverest of people alive in Westeros, and that she on occasion isn't capable to see the obvious. She can interpret the little brother thing quite litterely, as Tyrion is not only younger, but also smaller, whereas Jaime is only younger.
