Rejoicing:
Sorry to hear about your sister; i'll pray for her.
To answer your question, the uninstall XP option is only available if you are able to get to the XP desktop. After reading your posts again, i believe i was mistaken that this option is available to you because the setup program did not complete the installation of XP.
Now, to get rid of the message to choose which operating system you want, the easiest way is to edit the boot.ini file that the Windows XP setup created. You have to be careful doing this, however, because making a mistake in the file will mean you will not be able to start Windows 98 till you correct the mistake.
There are at least two safe things you may change in the file:
A. The default operating system should be changed to Windows 98 (right now it must be set to select XP setup).
B. The amount of time for the computer to wait before selecting Windows 98 should be set to zero.
Once these changes are made and saved in the file, your computer will seem to boot to 98 as normal. Then all you may want to do is delete the files that the Windows XP setup program created (that is, if you want to create more space on the hard drive to install programs and save documents, etc.). If it were me, i'd just delete the ARC path to the setup program in the boot.ini file, but i would not suggest you do this, as making a mistake will cause problems.
Of course, if you do find the Windows 95 installation CD (or set of floppy disks) then it will be easier, as i mentioned in the above post. Let us know if you want instructions as to how to edit boot.ini.
Blessings to you and yours,
Doc