First are you sure that she is getting fat or is swollen from retaining
water?
 
If she is eating as much as you think, I would try Totally Ferret.  It
fills them up better and they eat less.  I found this to help my one that
was getting too heavy.  They do have a senior which can be given to
overweight ferrets too.  You might try just feeding them separately,
instead of having it out all the time, also.  I am sure if they were
raised together, it would be very stressful for them to be separated.
 
Try to increase her activity.  Play chaise and tug of war with her;
anything to get her active.
 
I hope this has been of some help.
 
Phyllis and the four Wigglies, Faith, Hope, Charrity and Generosity
[Posted in FML issue 2419]