So, some cousin of your DH hates you. So what? We all at sometime in our lives find someone, distant relation, close work colleague or other who for some reason, usually unclear or unjustified, really loathes us.
Just have nothing to do with him. Do not bother trying to find out why. If your DH wants to continue meeting him, taking holidays with him, let him, just cut the cousin out of your life. Given the number of threads on Gransnet from parents and children who are at daggers drawn, which is tragic, cousins are expendable.
oldbatty I am inclined to support you.