A thoroughbred article from Bloodhorse back in 2014

https://www.bloodhorse.com/horse-rac...#disqus_thread

Some snippets

In truth, of course, the odds are stacked against a good runner being duplicated by a sibling. On average, full siblings share just 50% of their genes, meaning that siblings can vary widely in genetic potential. Add to this the influence of environmental factors—training, nutrition, general handling, early experiences, and so on—and it’s easy to see why more full siblings to good horses fail than make names for themselves.

Still, common sense would indicate that full siblings to a good horse have a better chance of being good runners themselves than half siblings, all other factors being equal and the respective sires being of roughly equal merit. As a test of this hypothesis, consider the produce records of the dams of horses named Horse of the Year

There were 434 half siblings to champions in the studied group. But if these results are any indication, perhaps breeders should have tried repeat matings more often. Of the half siblings, 261 (60.1%) won and 59 (13.6%) became stakes winners. But the [120] full siblings yielded 79 winners (65.8%) and 28 stakes winners (23.3%).

It should be noted that 120 and 434 are smallish sample sizes

I found this Comment on the article food for thought:

Even though the odds of getting a stakes horse are somewhat increased with full siblings I wonder if there is really an adequate return on investment if you are buying one of the full siblings rather than just breeding? The yearlings would sell for so much more than they would without a big name sibling, and I guess that's as much a reason for repeating the breeding as any - big money at the sales