Responses
To tell the truth, it seemed to me like he was testing him, too. But then it looked like he got really drunk - in that case, he probably thought of a reason for Mikados strange behavior, got upset and started to drink.
Also, that part with his senpai was something he did while he was wasted. Those brothers don't remember anything, while their cousins always do, as it seems. So it was most likely in the story to show that he not exactly takes anyone, but does realize it, when someone else tries to do him. To show that Mikado is special, so to speak.
I find it hard to believe that brother hasn't realized things yet. After all he hit the other guy for doing the same thing and then after he even initiated it. And then one of the next days he never told us what else was bothering him. It was like he realized something and then pretended to be drink just to test his brother out. Of course I could be wrong but no matter how many times I read this I always come back to that conclusion. the brother def is clued in.