Why do you say Renly knew about Cersei x Jamie? Doesn’t he just tell Stannis that it’s a good excuse?

Discussed here.

But here’s the short version:

1. Before he married her, Renly was scheming to get Robert to replace Cersei with Margaery.
2. Mace and Loras went along with this plan. 
3. None of this actually benefits Renly or the Tyrells unless Cersei’s children could be disinherited as well.
4. The only way that Renly could promise that to Mace and Loras is if Renly knew about the adultery/incest, which would make Joffrey et al. bastards born of incest. 

But, and this is key, the moment Robert dies without having divorced Cersei and disinherited her kids, Renly can’t admit any of this because it makes Stannis the indisputable rightful heir to the Iron Throne.