That's good advice, thank you. I would never bring it up, I'm more concerned about how I should react if she brings it up. Just say it was an unfortunate situation then move on?
You did not determine who from your DHs family was invited. He did. So stay out of this.
I don't know why an apology was owed to you. She stated why she and her DH were not attending. It wasn't necessary, but sometimes people feel the need to explain. When the RSVP was received, you and your fiancé should have shrugged it off. But instead you chose to see the comment as negative about his choice of who to invite.
If she brings it up, use one of the phrases suggested here.
The apology was owed because instead of simply declining if she didn't like the terms of the invite, cousin chose to make rude and passive aggressive comments about not being welcome. That was inappropriate and she ought not to have said it, and having said it, she owed Op and her husband an apology.
I think it was an error of judgement for OPs husband to have demanded the apology, as it just kept things going - but cousin did come up with a sort-of-apology so I think the route to take is treat the mater as completely closed.
If cousin brings it up, respond as Greenbird suggests - pretend that any comments about the wedding are what you'd expect from a normal person who had been unable to accept the invitation - mention that you missed them, and hope there will be other events. And then move on.
Smile, be polite, and fail to see an passive aggressive cuts - if she moans about you not visting, then ignore the moan, but respond about the fact you're seeing her now - e.g.
"you never visit, you're shunning us..."
"It's great we've been able to see you today, despite the sad reason for the gathering."
I would not respond at all to any complaints she makes about the wedding - I would not justify or explain the original decision on who to invite - anything she brings up, just keep the response to "we were sorry you couldn't be there."