He didnt lie tho
How would the MC know that?
Plus IT WAS A CONTRACT RELATIONSHIP. He knows they were both getting something out of it and the ML wasn't with him for love. Acting like this is some sort of surprise is stupid, it's like the author forgot their own story.
The ML manipulated the power balance in their relationship so from the MCs pov there is unfairness as he was completely honest in their communication and in general when you come into a relationship with people and act in good faith and show so, in a mutually benefiting situation, you expect the other party to do the same. So for MC it is now revealed that rather than him somehow failing to get across to the MC due to his own failure or his brother's meddling, it was thay ML is an asshole all along. So it is a surprise.
I can believe MC believing in his brother's words because at this point his brother is just running the victory lap around him. Even more so with the guy being bed bound in a clinic: he doesnt need to lie or deceive to win, he's already won, and the MC knows it.
"My brother is trying to kill me and is extremely sneaky and manipulative so I believe everything he tells me and allow him to isolate me from everyone else in my life." - The MC