Mother of twins here.
I'm with 14:51 (and also the other twin mom). Here would be my concerns:
- hiring a family friend is a recipe for disaster. You want to hire someone you can actually supervise/manage - not someone with whom you have an existing friend relationship, let alone extended family relationships.
- having someone live in all week - do you really want that? How will you define the job? Does she really understand what she's saying? (I'm, obviously, extremely skeptical.)
- caring for babies can be done extremely well by people without a ton of experience, but twins definitely ratchet up the difficulty and stress levels. It is not for the faint of heart and casual experience caring for other people's kids occasionally would not be sufficient for me to feel confident. Caring for twins is serious work. Hard work.
- do you know enough to train and manage someone else in the day to day realities of caring for twin babies? What if you end up having any NICU time, or any medical needs, etc...
- you will need to have back up care options in place - both to cover for your nanny (or whatever childcare solution you go with) and potentially just for some level of backup care for you. This person could be that backup care for you - have her help you in the first weeks, maybe do an overnight or two for you, give you a few hours off on the weekends, and be the person you can call when the nanny needs a day off.
Have you joined your local parents of multiples club yet? Do that and get to know some other mothers of twins and get their advice. Immediately!

They will be extremely helpful to you with decisions like this, and also with networks of experienced caregivers they can recommend.
Congrats on the twins!