In terms of ability to perform while pregnant, it depends. 16month twins are hard work- are they walking yet? If not, what will she do once she can't safely carry them?
I'm a nanny, currently 32 wks pregnant. Have not had any time off for antenatal appointments (scheduled them for my day off- I work 4 days), & only 1 sick day (caught d & v bug from my charges). however, I would say I've had an easy pregnancy and tiredness is just kicking in now at 32 weeks.
In terms of doing my job, I have a very long day too (sounds similar to your set-up) where I arrive 8am & leave 6.45. So far I am fine. My charges are 18months and 4.5 years. Thankfully the 18month old is a reliable walker so I haven't had to carry her for quite a while or do too much lifting.
In your situation I'd be most worried about lifting the twins up for nappy changes, into car seats, in/out of buggies etc. What does she do on a weekly basis? E.g. I am still out & about at toddler groups, playdates etc but long walks are off! We are doing more painting, playdough etc as I slow down. Is this feasible at your house? Is she a crafty nanny or does she have to do lots of physical chores? All this will affect you.
Worth having a frank & open chat with her sooner rather than later. She had to let you know about leave etc by 25 weeks. As others have said, I don't think she is entitled to SMP but can claim MA. The government reimburses you for this so you are not out of pocket.
We (my employers & I) planned for me to stop at 34 weeks- so far this feels about right. Long days are getting harder, especially running around after toddlers!