The only fix ay be to put cross bracing in the cabinet to support the sink from the bottom.

2 cm granite normally uses the plywood under-layment to support the sink lip.

3 cm granite can have bolts placed in slots in the underside of the granite to hold the sink clips.

Hi I have had to do this same job twice so I have a system down pat. If you haven't started yet post again and I'll jump on and let you know how to fix your problem once and for ever.

I used the brickeye method (this guy is always right) and epoxyed a piece of pylwood across the undermount lip and screwed it into the front and back of the cabinet as well as used silicone around the sink itself.I clamped everything together for 24 hours. The biggest chore was cleaning off the old silicone from the granite. Scraping and Acetone worked best, then cleaned with alcohol. Whole job took 2 hours. It's been 2 weeks now and all is fine. But p10rs if you have another way or better way, I am all ears.