I have two stepdaughters, ages 11 and 14 when my daughter was born 3 years ago, and we have a similar relationship. I do refer to them as stepdaughters and they call me by my name since they have a mother, are with each family half time and they were really raised by their mom and dad, who did a fantastic job. I know I'm part mom/part friend but really they have a mom who parents them and is great.
we had my daughter around Christmas and told both girls one day in the car by asking if they wanted to do something special for Christmas, like have a baby sister. they were so surprised and it was so out of the blue they loved it.
I agree with another post about involving them in the process. we actually moved to a larger house, closer to their mom when I was 6 mos so we decorated all three rooms, bought stuff for them and their rooms like we did for the baby and they were a part of it all along. they came with us for the ultrasound around 20 weeks, attend baby showers, first stop on our way home from the hospital, etc. It was really nice to keep them involved along the way and they appreciated the attention on them as well as the baby.
the biggest problem I had when people found out I was pregnant and had preteen/teen stepdaughters was the overwhelmingly common comment "how great for you, instant babysitters!" we made very sure they were sisters, not babysitters. if they wanted to play with, hold, feed, change, bathe their sister, awesome, if not, then they weren't forced like it's a chore. the youngest didn't even hold her except while sitting on the couch for months because she was so nervous about dropping her and how fragile a newborn is. flashforward almost 3 years and they have a great relationship with sister, help out all the time because they want to, give us gift certificates for "babysitting" as a gift and just are there as an extra set of hands which I know has made being a parent so much easier for me.
I know she's not into jewelry now, but she might be in a year or two, our tomboy 11 year old is now a freshman in high school who takes just as long getting ready (make-up/hair) as her older sister. the necklace or other piece of jewlery might be really meaningful and more valuable even if she doesn't wear it now but does later.
good luck to you, it's great she is so excited about it.