I would be frustrated in your position also. I can't imagine having an established reef tank with fish and corals, to include SPS, and then an Octopus moving in to my surprise. Even if it does not eat fish and inverts, it could damage corals and rearrange rocks easily.
I would enlist a helping hand or two and carefully find a way to safely remove the Octopus from your reef and then take it back to the store where you got it. I would look for a full refund and I am sure you will find an ARC sponsor store very easy to work with.
I would also put your brother on lock down and maybe help him setup a separate predator or dedicated species tank he can mess with without harming your reef - maybe you can enjoy that too and protect your investment in your reef at the same time.