It is stated some were, I think in Appendix A, that a special choice was granted Arwen, that she could remain in Middelearth in the youth of the Eldar, as long as Elrond remained, but he left for Valinor she had to make her choice either to leave with him and life on as one of the Eldar or to remain in Middleearth and become a mortal.
In of the Letters Tolkien said that her brethern would natrualy have the same choice but that they could in what ever way delay it. That means that we can not be sure that both of them choose to be mortal also we here that they stayed behind when Erond left.
Respectfully
Findegil
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