I'm replying to this old thread because I couldn't find a newer one. I had exactly this same problem, but I was able to fix it quickly using something I found on Amazon (Never Fall sink repair kit) which might not have been available 6 years ago:
In my case, the original installer had put in two wood blocks screwed to the left and right cabinet walls, which kept the sink from collapsing completely. But the sealant failed and the sink was sagging in the middle. I cleaned off the old silicone caulk as best I could (denatured alcohol, scrubbing pad, scrapers, elbow grease) and installed the two struts that came with the repair kit. I put them at the back and front of the sink, rather than the sides, because the blocks were already doing a good job of holding up the sides. I applied new caulk, extended the struts and locked them in place, and it seems to be as good as new.
I didn't use the device that the kit provides specifically for the front of the sink. It would have been impossible to drill a hole in the front of the cabinet framing because the sink is too deep and gets in the way. Instead, I installed one of the two struts onto the center pillar of the cabinet.
I hope this helps someone! My only worry is that without removing the sink, it's very hard to get all the old silicone caulk off the underside of the granite countertop, so the new caulk might not form a good seal. Time will tell.