you don't say what's in the bottle, if it's formula then that might be
the problem, if it's breastmilk, then you probably need to look at
things like position and how you are holding the bottle - could she be
taking in air - it does sound like something about the feed is making
her uncomfortable after a period of time. Try to have her as upright as
possible and make sure the nipple is full of milk - unfortunately this
can mean a waste of milk as you can only manage this through the whole
feed if there is some left at the end. Another possibility is that maybe
you are overestimating how much milk she needs in a feed and that her
discomfort results from you encouraging her to take more when she's done
and her tummy becomes over full. You could also try burping her a few
minutes in.

Give your wife a pat on the back for breastfeeding premie twins...

cheers
Anne