Renly didn’t know anything about incest. Idk why people seem to think so, but why would he hide it? He’s not that stupid. Robert would marry Margery if he needs Tyrells – and he would need them, their money and their army, if he’s going against Tywin. Renly could’ve gotten what he wanted without doing anything. Hell, it was easier to quietly kill Stannis (Tyrell style) if Robert refused to marry rather than bother fighting a war. No one will fight to put Shireen over Renly on the Iron throne.

I’m not sure how much to put into this response, because I’ve written about this here here here here here, here and here. 

But to answer your questions:

1. Renly hid it, because until Robert signs on the dotted line with Margaery and the Tyrells, revealing the truth would have made Stannis the heir to the Iron Throne. Renly doesn’t want that, nor do the Tyrells.
2. If you think about it, it’s the exact same reason why Littlefinger and Varys withheld the same information, despite ultimately intending to bring down the Lannister regime. 
3. Rather difficult to do that when Stannis has withdrawn to Dragonstone and started raising an army and navy to protect himself.