Aside from having them be related, the most obvious way to get them to interact would probably be to have Cadance serve as Diamond’s babysitter for a night, though I’m sure we can get more creative than that. How about Cadance going on her honeymoon only to find Diamond is staying at the same hotel and refuses to leave her alone? How about Diamond contracting a disease that makes anything she touches turn to crystal, a disease which Cadance is inadvertently responsible for? How about Diamond visits the Crystal Empire with the ill-conceived notion to “run for Princess”?
Yes, but everyone hates DT