They are sure that Celine's other brother is also their enemy. But this man hasn't had a need to do anything yet because the first brother's biological warfare plan is enough to deal a devastating blow to the Golden Hammer gang.
If the disease is let loose, not only will the bulk of the members of the Golden Hammer gang be killed. The leaders of the gang will still be alive, but the gang will have to disband because no one will want to work for a gang that has broken the rules before and paid the consequences so severely.
No godclad will want to work for the Golden Hammer gang and put themselves in a situation where they can be killed by some disease. So it would be a devastating blow to the gang.
It is no wonder why Celine's other brother doesn't think he needs to prepare anything to punish the Golden Hammer gang. Unfortunately, their other enemies don't know about this plan and are preparing their own devastating plan.