Did Arwen know?

[Elrian]

Elrond only decided to leave Middle Earth after the ring had been destroyed. Right?

Elrond would leave eventually, and his children would have to choose, he and his children knew this long before the War of the Ring. Elrond was told of this at the end of the First Age, when he himself chose to Eldar. In that quote Arwen says maybe, not that she will have the same fate. If she forsaw the love between her and Aragorn at their first meeting, you sure can't tell by her words. If there had been no love between them or no marriage, the choice would have still been hers to make and she still could have chose to remain and be mortal.