Nana, thinking of you (((hugs)))
Wouldn't it have been wonderful if DIL could have come to you beforehand and said, "Nana, I'm occasionally stressed out over these kids. Is this normal?" You could have reassured her that she's a wonderful mother and that all of us moms have had rough days. Then you could have offered to be there whenever she felt she needed to call, either to talk it out or take the kids for a couple of hours. I just know you wouldn't have judged her or expected her to suck it up and go it alone.
Why do so many of us DILs/MILs feel judged by our MILs/DILs? Why aren't we sisters in all of this?
You know, maybe that's how you can handle it..tell DIL it's normal to feel like losing it every now and again, parenting is hard work and you understand! But that she can always count on you to help if need be, either to listen or take the children and give her a bit of a break. Would she be amenable to your expressions of kindness and understanding or would she feel you were patronizing or judging her?
Nana, your issues are no less important than any here. I hope no one hesitates to mention something because they think it's not as critical as what someone else is going through, because you never know when it will help someone else.