It certainly looks sounds the same woman.
Do you mean that the transcript of marriage cert. Sydney 1865 has parents for Catherine, but not for Frederick?
Do you have names of witnesses on both marriage cert. that you have...1865 and 1869?
Those names might lead to close family relatives for Catherine to support her being sister of your ancestor.
Or family of Frederick, the better to possibly locate a death.
If the first marriage ended in divorce you would expect to find mention in the newspaper. I wonder if that marriage was annulled?.
Where did Catherine's sister, your gggrandmother, marry/live/die.
You might find mention of Catherine in the death notice of a sibling, years later?.
What do you know of the Driscolls...did any of this family live in north Queensland? Might account how/why Catherine goes such a distance from Sydney.