Given that this kind of situation, while uncommon, happens every now and then, financial aid departments should know how to handle it. It can probably be accounted for on the FAFSA/Profile when you indicate how many people in your family will be paying tuition in the next year.

I disagree that this is common enough to expect financial aid departments to be used to it. This is certainly a special situation, and if you call the financial aid department(s) you're wondering about, they'll certainly be honest with you and also make a note of your situation so they can treat it with the attention it needs. I think the FAFSA will help take your twins into account already, but just to be safe and get that extra help, I'd call the offices.

It's never a bad idea to call financial aid offices to discuss financial aid packages, even if their aren't special circumstances.