It's not about "fair" so much as it is about consistently sticking up for the principles you establish. If you didn't let your older daughter go on an overnight school trip in 7th and 8th grade because you were uncomfortable with the supervision, the same evaluation process should apply to the younger ones. If something has changed in the way the kids are supervised and you are now comfortable with what's going on, then it's ok to allow the younger one to go. And of course you'll have to evaluate the third one's requests on the same basis when they come along. But don't use "your sister couldn't go so you can't either" as a excuse for sharing the real reason why you won't let her go if that's what you decide. Your daughter is correct: she is not her older sister, and should not be denied something just because her sister didn't get it. You have real concerns about her safety, as you did about her sister, so explain those concerns and stick to your guns if that's truly what you want to do.
Now if the problem is not safety but money, there's a logical way to resolve it for both children: tell them that if they want to go, they'll have to raise the money. Both of mine raised money for a trip they wanted at some point as did many of their classmates. Raking leaves, mother's helper, or doing the door-to-door fundraisers that most groups use for trips like this is not beyond a 7th grader. If you're really ok with the safety arrangements and just concerned about the money, help your younger one to fend for herself. Oh, and what about the older one who can babysit, tutor and more at this point? Encourage her to raise money for her trip too!
Bottom line: it is certainly not "unfair" either to keep the younger one home or to let her go. Just make sure you clearly explain why you kept the older one at home and why you are letting the younger one attend if that's what you decide to do. Or why the younger one CAN'T go. My children didn't always like the fact that they couldn't do something, but when I explained it as a health and safety issue, there was grumbling but no rebellion. Children really do want to be kept safe by their parents.