Just say, "You know, I've often though about getting married, but when I look at the mess a couple of sad prats like you have made of theirs it puts me off. Nothing personal mind, just an observation".
I wouldn't say that, just respond with: "You still married then?", with a wry, confident, teasing smile on your face that makes it clear that ^^ is what you're really thinking. They won't like that. It's ambiguous enough to put them on the back foot. Then keep it as your stock phrase, just as they have theirs for you. Any time they make 'those' comments, just say "You still married then?" All the fun will go out of it for them, they won't know how to 'regain the upper hand', they'll stop.
I agree with the overriding importance of trying to avoid screwing up your mum's birthday, but she chose to invite them, despite presumably having been witness to their behaviour in the past. She, as well as they, needs to know that you won't be bullied, and if necessary you're quite prepared to give as good as you get.