How did your vet come to this diagnosis? There are specific tests that determine this disease. If it wasn't determined before puppy's death I'd think the only way you'd truly know was if an autopsy were performed. Lacking either of the above it sounds like it would be safer to retire the two you are breeding. LS can go back 10 generations....
This is from the leading authority on liver shunt -
http://www.vet.utk.edu/clinical/sacs/calendar/