they both at fault bc
Mc - dumb asf and wants to date his brother to the point of agreeing to practice sex w ml. its understandable that he still couldnt let go of his feelings bc they became family at an older age but its common sense not to pursue him for that reason
Ml - dumb asf for not expressing his feelings in a healthy way. he's technically coercing mc to have sex w him and exploiting mc's naivety. even if they grew up together and became inseparable that doesnt mean he's entitled to receive mc's love
in conclusion theyre both dumb asf and need to communicate w each other. also ml needs therapy bc wtf is wrong with his line of thinking

The best friend really need to grow some self respect. I understand you love mc but you should also need to know when to give up ..I really hate mc, if I have a bsf like him I would already be married to him