All good ideas. A few negs:
other sister (weird one) lives CSP and we've been a few times
All been to Spain (still a possibility but time limited)
DAB is tired of beaches (not sure how she'd feel about the dark sand ones)

Colonial Mex, CR or Nic are contenders, for sure. BJX is kinda odd to access--Puebla is few hrs bus ride from MEX airport. Belize jungle still option, too. Istanbul was so simple to plan--this one is tricky.

Keep the inspiration coming. It's really hard since we don't know timing yet. I'm going to Africa end Arpil, ones going to PHX, and one babysits grands alot. On top of that 94 yr grumpy old man (beloved grandpa) refuses to die or treat his med condition (don't blame him for either!) so that's tenuous.