How about just agreeing with your grandma that it's hard to raise children, and harder to raise multiples. Agree with her on that much and don't argue with her on the rest.

Frankly, I'm finding myself defending your Aunt. Your Aunt did not choose to have your sister's babies. Why is it up to your Aunt to "help out?" just because she's the closest geographically (and the next state isn't like down the street, but even if she was down the street...it's not her responsibility)? What about the other family members? If it's truly a crisis, people move, people take turns, people do what's necessary.

However, it does not seem like a crisis situation. Just normal, hard, multiple children thing. How many kids are we talking about? There are other support systems for families of multiples. It's not easy, but I don't see why it falls to your Aunt to be the family support person. Afterall, she has a life, too.

Back to Grandma, she may truly believe that your Aunt has bent over backwards. You don't have to agree, but as you said, she's stubborn. Perhaps her support of your Aunt is a backsided way of being mad at the rest of the family for not helping out more.

Best of luck to you and your family.