Is the disk readable in the PC's floppy drive? (i.e. is it a file format problem, or a disk format problem)
The latter *may* be solvable with one of the "alien" programs from years ago that could read alternative disk formats.
If it is a file format issue then you may be able to write some quick hack to change the format. If possible, a test document containing known text would be useful.
Somewhere mid-way is writing a program to pull raw data from the sectors of the floppy and manually reconstructing the file(s) found.
From a quick Google search it appears that it uses a proprietary disk format, thus it might be very complicated or expensive for you to do the conversion yourself...
Likely best to just send it out to a company like http://pivar.com/ that has made it their business to figure it out and convert it for you...
I'm sure it is possible, heck most of the data recovery software that accesses disk at the raw data level could almost certainly pull it off if it could be configured properly... And there is likely software out there that will do it, but finding the software might be a hassle and or costly... I would guess back in the day someone wrote a DOS program or two that would accomplish the task quite easily, but most of those programs are likely collecting dust in long forgotten archives...
Is the Brother still functional? Have you considered just printing out the files and using OCR software to scan them back into a modern format? Clean printouts should scan in perfect with new OCR software...
You could try looking for people on the internet who collect them (I haven't looked, but there are people who collect *everything*).
If you find such a group, they would probably be the best source of information, and possibly working machines that could be used to save the information in an alternate format.
Have a look at the "cpmtools" package in linux, and try to find out which format is used by
your device. If it something a standard PC floppy controller can't cope with, you may try
to build a µC circuit that accesses a floppy drive and sends the data to the PC via RS-232
maybe.