Another question, is your DH your DS's Dad? (hey, it's possible!)
If I were you, I would start pro-actively telling DH exactly why it wouldn't be 'fun' for him to be there. First off, there will be lots of talking about boobs, wazoos, incisions, stitches, various fluids, etc. If she's nursing, the first 10 days are the hardest (never mind with TWINS!), so she'll need to be comfortable and will likely be exposed a fair bit of the time. Me and my Dad had a hard time of that at first, he couldn't just leave the room every time I nursed, so we developed a 'no eye contact' agreement and that was MY Dad. Also, when my parents stayed with us after DD was born, my Dad actually had to leave early, because he just couldn't stand the crying (me and DD!). Not that it irritated him, it broke his heart.
When you're there, you absolutely have to give DIL a 'get out of jail free' card. She will not be herself, and you should NOT take anything personally. Let it roll off your back, and even imagine it in your head that you're giving her a business type card that says "I know you don't mean that." if it helps you.
When you're there, try and do everything else for them (that you can) to leave them time and space to take care of the babies. Do your DS's chores, so that he can help her with the babies. Ask them HOW they want stuff done, whether they want you to put stuff away or not. For example, wash the dishes, but leave them piled nicely to be put away in their proper places. Same with laundry (except for skivvies - let DS do the skivvies load). Offer to change the beds (if you can).
Even if you raised hundreds of kids, pretend that right now, you know nothing about it - it HAS to be their way, they have to figure it out for themselves and let them tell you how they want things done. I don't know if you have other gk's but times have changed, DIL has probably done a lot of research. And even more importantly, she has an image in her head of how it will be, and it's important that you don't disturb that. I mean, sure, it will be disturbed, because nothing ends up how we idealize it, but really, you don't want it to be YOU who does it.
Bring yourself a book and let them spend some time together as a family too. Remember to wash your hands before touching the babies, don't even wait to be asked, just do it. Don't take flash pictures of them, it's too bright. Don't wear strong perfumes around them. Don't leave lipstick marks on them.
I know this sounds like a lot of rules, but really, you're setting the tone for a whole lifetime with those kids, you can suck it up a bit now, while their Mom is at her most vulnerable, in order to be able to enjoy the whole rest of their lives.