I'd ask him straight out:
"Honey, I'm horny...right now. Are you in the mood or can I go bang your brother?"
The initial shock might make him think, but if he's not in the mood, he's not in the mood, forced sex tends to really suck.
Also, this only works when you would be okay with the same situation when the roles are reversed. If you have a sister and you weren't in the mood for sex, would it be okay for your husband to have sex with your sister? It isn't fair if one person has all the power.