Maybe Renais kept Grado sacred weapons in their...custody? Something to prove they won the war. Maybe at the time Renais stated they would give them back when Grado proved they could be trusted or something? 
Or maybe...just maybe this; an idea I think I prefer. Maybe at the time Eirika, who loved Lyon quite a lot if I recall correctly, decided she'd keep the weapons until she could give them back to a real heir to Grado's throne. Either it'd be an hidden member of Lyon's line, or a hero that arise from Grado at some point in the future. However her statement got lost as time passed and her descendants started to see the weapons as a sort of war trophy, wanting to keep them, which did not help relations between Renais and Grado at all?

As far as as the Jehanna weapons go, I think I'll make my heir take at least one with her. It would be important since, on the day Joshua's mother gave him the weapons, it was a way of entrusting the country to him.