2 points.
First, Misah matirs even in this life, certainly after Tchias haMesim. The ONLY question would possibly be by Yibum, if the first brother has tchiya before she and 2nd brother dies. Nothing was matir her, so perhaps you could ask by yibum.
But your whole question is based on the wrong assumption that there will be marriage at that time.
The entire purpose of marriage is to have children, and the entire purpose of having children is for continuity of life. In the time of Tchias haMesim, and Bila haMaves laNetzach, there will be no death, and therefore no continuity of life.
(Really, if there is no death, there is no life, as life is defined by death, but that’s a separate point).
A proof to this is that which chazal say, bo’u vnachzik tovah la’avosenu, we should appreciate our ancestors who sinned by the Golden Calf, because if not for that, they never would have died, and we would never be born.
No death, no children. No children, no marriage.