OK, that's A reason, CV.

Is that A reason why you would not do something?

If your H is crabby for ANY reason, does he take it out on you FIRST and if you are not available then he takes it out on S?

Or does he take it out on everyone?

And in the end, if he's crabby with S, then set a boundary there.

"H, if you are upset about something, do not take it out on S."

It is his relationship with S that will be harmed. That's his problem.