It's atypical to see mitered siding like that, at least in an exterior application. Usually you'd have a vertical trim board up the corner, to which the siding abuts. Then you have a small corner to caulk, which is more effective than trying to seal end joints. The same would be true at the inside corners at the outer edges of the bay.
There are many ways you could handle this. You do need something other than caulk here, though--something that overlaps the siding for better protection.
Since you have nice flat tongue-and-groove siding (as opposed to lap siding with its sawtooth shape), I would suggest a trim board overlay. First, though, run rubber window flashing tape down the joint. Cut it to just smaller than your new trim.
Then fit and attach boards a bit thinner than the window casing. Either mill a single angled board or bring two together with a miter. It won't much matter since your flashing tape does the waterproofing. (These could instead be the aluminum strip you mentioned. If you go that route it should be hemmed on both edges.)
These could run just to the top of the window casing or all the way, depending on your preference. I'd paint them siding color, not trim color, so they aren't so conspicuous. You'll have to think through the details here since the siding corners are not centered on the gap between windows. The trim might be narrow between the windows and expand to some uniform width above them.
Caulk the edges of these new boards as appropriate with minimal beads. All good.
