YES! YES!! YES!!! YES!!!!! Your babies are not typical babies. They are twins and they are preemies. Relatives can think you're rude or mean. You will learn to get over it the first time you let someone 'break' your rules and the twins get sick or are up all night and you AND dh are up all night. I feel so strongly about your need to set firm, strong and inflexible boundaries at first that I might just blog on it.

(Unless you mind.)
You can politely tell them, give them a reason why, and leave it at that. If they insist that you're ridiculous--tell them that you are more than happy to let them hold, wake, etc. the baby as long as they agree to come over and do the night shift and/or come to help with the cleaning, cooking etc.

No one, not once took me up on my "offer."
I guarantee you that if you start second guessing yourself, and let someone who has a little cough hold the baby, or let people come over and start holding the babies non stop--you WILL regret it. (ahem. . .off my soap box now)