This is probably the most important rule of the bro-code, one that cannot be broken AT ALL.
The moment your bro gets out of a relationship, the girl he was dating is as good as dead for you. Sure you can still have conversations or stay friends with her, but dating/hooking-up/flirting with her is completely off-limits.
6. Your friend's sister is also out-of-bounds.
Even more so than your bro’s ex-girlfriend, his sister is completely out-of-bounds.
No matter what, you can NEVER hit on your bro’s sister. In fact, this is considered to be the most holy rule of the bro-code, one that, if broken, can even end friendships.
The only exception ever made was when Chandler and Monica got together and told Ross, yeah, well that went differently.