Thankyou for your replies......I also posted this problem on the yahoo groups nail tech board. We found the answer! So, here is what reply we got, it may help someone else:-
If the split is just off center of the nail plate and is going from
cuticle towards free edge with small 'feathery' cracks projecting
towards the side walls it sounds as if she may have a condition
called 'Median Nail Dystrophy'
In advanced cases the split may expose the nail bed below.This
condition was first identified in 1928 and is similar to Habit
Tic.Most research points to a nervous habit as being the main cause
although can be caused by other types of trauma. Once it appears it
can take many months or even years for the nail to return to normal.
In many cases it can leave a ridge.
If your sister is not a 'picker' it may be a good idea for her to
visit her gp to rule out other causes . As for treatment . If the
nail bed is NOT exposed and the condition not adavnced then you
should be ok to apply a gel or acrylic overlay. If conditon is
advanced it is usually recomended to use cuticle oil daily along
with massage to help promote a healthy nail growth
I have one client with this condition in a mild form and although it
has not improved it has not got worse. I recently had a new client
come to me with it quite badly split and it was actually weeping . I
sent her to her GP and he said that there was nothing wrong with the
nail!!!!!!!
Hope this has helped you