The xX kid is right. She needs to back up her files first. Invalid sibling happens when the disk catalog (loosely speaking-- pointers to where the bits of the files are) is corrupted. In most cases, this results from underlying hardware errors (her drive may be gradually failing). But it's also possible that it was a bug on MacDrive's part, which corrupted the catalog.

Whatever. Point being, can anything be done? It *may* be possible to fix it with a commercial tool like DiskWarrior. She can also try the following (adapted from: http://www.macosxhints.com/article.php?story=20070204093925888):

1. Boot into single user mode. Hit F8 and type -s and hit enter.
2. Type /bin/df and look for the drive that has her data. Hopefully she'll see something like this:
   
   Code:

   /dev/disk0s2    81875392 76037824   5837568    93%    /Volumes/DATA

3. Type /sbin/umount /dev/disk0s2 replacing disk0s2 with whatever disk her data is on. Now, it shouldn't be mounted, so this is just in case it is.
4. Type /sbin/fsck_hfs -r /dev/disk0s2
5. Hopefully this should fix the problem.

Apple's man page for fsck_hfs says:

-r Rebuild the catalog file on the specified file system. This option currently will only work if there is enough contiguous space on the specified file system for a new catalog file and if there is no damage to the leaf nodes in the existing catalog file.

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MacBook Pro - have allergy to nickel in the aluminum casing. So my kid gets an expensive toy!

Gateway MX 8738 - Retail, vanilla Snow Leopard 10.6.2 (thanks kizwan!) with Chameleon RC4, modified DSDT. Upgraded to Core 2 CPU (easy to do). Upgraded to 640GB drive. Everything but SD card working. Minor niggles. GMA950 with QE/CI and *no* artifacts.

iMac (luxo/lamp) G4 with Tiger.