Connie did not know about Fredo in adviance, but she did figure it out over the years, and even forgave Michael for it in GFIII, where she was VERY involved, both in the unathorized hit on Zasa, ands in the planned poisoning of Altobello.

To that end, I wonder about how that planning went down....as of that time Vincent was in charge of the family, and it was Vincent's voice giving the orders to Neri and the rest of them to go kill the Archbishop, etc.
I think they knew Altobello could not be assassinated in the open because he had protection, so I suspect it was Connie's idea to slip him the poisoned cannoli, and she did so with Vincent's blessing. This raises the question:
DID MICHAEL KNOW THAT CONNIE WAS GOING TO KILL ALTOBELLO?