Prediction: If they really truly did not want to have children, they will not fall in love with the baby until he or she is here. Then all bets are off. :-) She will probably be ambivalent throughout her pregnancy -- the hormone swings are tough enough on women who WANT babies -- and her husband will probably be in denial. My husband wanted babies in the worst way and still didn't get a real grip on the situation until the minute he witnessed the birth.
As a mom who falls much more toward the cocktail or jello mom end of the spectrum rather than the helicopter mom end, I think you're right to try to contain as much of your enthusiasm as you can. Just try to be calm and practical and helpful in whatever way she seems to need help, which could change from day to day, unfortunately. :-) One concrete way you can help her is to find ways to help her wade through the endless details of what a baby needs and the completely overwhelming variety of stuff that's out there. When the time comes, find a way to help her register, for instance -- maybe online? Start compiling a list for yourself -- not to give to her until she asks, probably well into the second trimester -- about what she needs and what she really doesn't, and what was indispensible to you with your baby. Don't mention it until she does, but be prepared to help her develop a plan for the nursery, and whether or not she wants to breastfeed, and what sort of L&D she wants, and how to pick a pediatrician, and what she's going to do for work after the baby comes (I am making the sweeping assumption that in lieu of babies, she's been a career girl).
Most importantly, accept now that her reactions to EVERYTHING will probably be 180 degrees different from what yours were. You seem to have a good grip on this already, but it's worth stating that her reactions will not be wrong, just different, and that's OK.
And finally, be prepared that she will probably procrastinate on things. She doesn't have to do things at the same time you did or her friends did, but she does have to have some barebones stuff in place before the baby gets here. Your job as experienced mom and sister is to make sure she gets those barebones things in place before week 37. I say this with experience, as I did procrastinate on some things and was then nearly left in the lurch as my first baby was a full week early. :-)
Sorry this is so long; hope it helps!
J.
PS. I just absorbed the bit about her feeling like her life is over. On the one hand, this is good because it says she understands that the life of a mom involves a lot of sacrifice and responsibility. On the other hand, be prepared that she might do something really stupid (I'm not talking about hurting the baby -- something else like have an affair or take some extravagant vacation she can't afford) or think she needs to make some life-altering change if she feels like she needs to prove to herself that she did a good job of living before she gives it up to become a mom. Try to help her curb this tendency by pointing out that she doesn't have to be like other moms; she has to be the mom SHE IS. She is still a person, and her brand of motherhood will take on the characteristics of her personality, and she may actually enjoy it!