OP - 21:10 here (twin mom). Yes, you need to get over the superwoman thing quickly! You could do it all, but the price you'd pay over time isn't worth the bragging rights.
I'd suggest thinking big picture w/ the neighbor you'll be hiring - think about the maximum she might do for you and build it on that, not the minimum of just helping w/ the kids. I have learned (truth be told I'm still working on...) to ask for help, but I've also learned to fully accept it and not minimize the value to me. So think about whether having your neighbor help could include solo care for the kids for an hour or two on occasion, moving/folding a load of laundry on occasion, handling a meal or two alone on occasion. The value of an hour totally baby free is more significant sometimes than 3 hours of having someone else around to help out. You might find that being able to leave the house and go shopping (for anything, but especially clothes or shoes for you or a holiday/b'day gifts for someone, etc...) or for a pedicure or a lunch w/ a friend, or a matinee, etc... will be incredibly restorative to your sanity. Twins are wonderful but they are hard, hard, relentless work.
So it is well worth the money spent to have help like this, but it's also worth making sure you get the most value of it and I would really encourage you to think about getting the best support/break for you - not just the minimum of having someone else around (though that certainly has value too.)
I'm not saying you'd leave her w/ the kids routinely, but just knowing you could do that on occasion has value (and framing it that way for her at the beginning preserves the option for you).
Good luck!