With the facts as we now know them.
What I am interested in is whether - based on what we know now - people would prefer to have one elderly relative injected twice and the other one unprotected, or both elderly relatives injected once each. For me, I would go for both once each, but I'm interested in other views.
With no expertise in this, I'd be more comfortable with both of them having one jab each (even in the unlikely event of the protection "fading"). Get as many people protected as possible, to get over the hump of the new virus variation. If would obviously take a long time to compare the efficacy of the 12-week gap versus the 3-week gap, but I'd hope there is some live trialling going on. Longer term, I imagine we'll be getting vaccinations on a regular basis as per the flu.