Deanna's marriage to Fredo was such a mystery. We know that they were obviously married at the time of the communion (she wanted to get closer to Michael-that tramp!). The last visuals of Deanna and Fredo is when Johnny Ola called Fredo to seek more info about the Frankie/Michael deal.
The last mention of Deanna is Fredo's confession to Michael that he wished he married someone like Kay at the cafe in Havana.

Deanna did not attend Mama's furneral. My personal belief is that after she learned about Fredo's estrangement with Michael: she took a hike!