I've scanned the threads in the past but won't have time to re-read them today but I'd like to go and buy dd some more bras. Her measurements were 26" under and 30" over. While I was measuring her I checked myself and I measure 30.5" under and 34 over - 35 when leaning over. My boobs are quite "empty" on top if that makes sense.
With a D cup even on a 26 rib, she still needs support. Growing breast tissue is very delicate and needs proper support, which is why a badly fitting underwire is a no-no, but a properly fitting one the best solution.
Now unfortunately 26s aren't really made unless custom, which probably isn't worthwhile for a teen who will grow quickly. Unfortunately 28C is also as rare as rocking horse poo, but M&S do a few and there are reports that they are running small this season. Mimi Holiday/damaris also do some 28Cs which would have roughly the same cup volume as the 26D she really needs. If you need to adjust, you can either get a rixie clip (ebay) or simply remove the section with the clips, take a small amount of fabric out of the band and then reattach