I think she did know that Michael killed Fredo and she has come to accept that. In the first film, she is just another woman in the family, by part ii, she wanted revenge from Michael but at the end, she resumed the "typical" woman role and wanted to be Michael's side. She understood the business and by GFIII, she kind of gotten herself a bit involved by telling Michael to accept Vincent. And then ordering Vincent to kill Joey Zasa. Then killing Altobello.

Connie develops plenty throughout the trilogy -- that's why sometimes I find her character so fascinating.