The fact you're concerned enough to ask seems to indicate you'd be willing to work with your sister in making the ride a successful one.

My stoker and I did a metric century last year (new course for us), and it was probably our 10th or so ride on the tandem, including a really shaky 2 or 3 mile first ride (first ride in a year for stoker after the her last ride, on a single, resulted in a heavy crash).

As I am more fit than my counterpart, I offered the following tips to her:
1. Spin easy, save your legs. Cramping halfway through doesn't do either of us any good. I work hard on the hills, cruise everywhere else. She pitches in when it gets hard, and due to her taking it easy for most of the ride, at the end we can really go hard. Feels better than crawling home.
2. I say to her tell me when you need a break - and I try and offer every 5-10 minutes. We just stand and coast, one leg at a time.
3. Let me know when to slow on the descents. As someone who wants to go as fast as possible, the concept of braking on a descent is almost foreign to me. However, I realized how scary "being scared" is when I got scared on a big descent I didn't know. I try and recall that feeling whenever I feel like we're going too slow. Curiously enough she is comfortable going just as fast or faster on the flats, but I think it has to do with the fact that when we stop going hard, we'll slow immediately.
4. I read out cadence (I haven't mounted a second computer yet). I quickly learned the comfortable cadence and try and keep to it (varies based on hills, wind, fatigue, etc).

Although not critical, she wears a HR monitor for herself. I was surprised at how hard she worked even when "taking it easy". I can find no fault in her effort level, and the HR monitor put some objective numbers behind that thought.

Fit is critical. Her shoes, pedals, seat, gloves. I find the cranks to be much wider on the tandem than my road bike but there's nothing to do about that. Seat height and setback are most important with knee health so make sure you can replicate both your positions on the tandem.

Make sure you have a suspension post for the stoker - it'll help when you miss that odd pavement crack that seems so insignificant but bounces her right off the seat.

I think if you can go for a 10 or 15 mile ride the day before, check fit, gears, brakes, and discuss anything that pops up about riding the tandem, you'll be fine. You might have spots of difficulty here and there but I think that's what makes doing a century challenging and rewarding.

cdr