As we know the Sussexes are expecting a daughter this summer, depending on the timing, it just may not be feasible for Meghan to travel internationally with a newborn infant. In this case, it very well could be only Harry that attends. I don't believe attendance at something like this would solely be affected by feuds or animosity between family members but 95.5% centered on the *why* this event is happening in the first place. To remember and honor their mother and grandmother of their children. That's the top priority here.
I don't have any expectations but my guess is that the goal is for it to be both of her sons side by side unveiling it. They have until July to figure it out.