Cat Attacking Issue

[Obi2017]

We have five cats.  Two boys and three girls.  

Two boys were adopted first, and we adopted them at different time.  Two boys are two years apart(8 & 6), and they have grew stronger bond between them over time.  Of course one boy is the leader, and another one is the follower. General speaking, two boys are spending much more time together and they do not real like to interact with the girls that much, especially the leader boy, he only snuggles with his brother, and he did not like girls get too close to him.  The boys like each other and do not real fight.

However, whenever the boy (follower) vomits (not very often), another boy (leader) would go attacking/chasing and beating him very aggressively.  This kind of situations have happened many times.  I tried so hard to stop the leader boy beating his brother but he always would get him.  Fortunately, the attacking would only last for a minute.  Maybe because I got involved.  Anyway, I just could not understand why a nice boy, the leader, who is acting smoothly on normal days, but would suddenly became a aggressive attacker after his brother vomits.  Interestingly, he does attack girls if they vomit.

Any suggestions and advice would be highly appreciated!