Writer's serendipity happens a lot. It's the phenomenon of finding exactly what you need at some point of a story. Sometimes it works with props. Your character needs to write a note and you remember mentioning a notebook she always has in her backpack. Or you need to put someone out of touch. Hey, remember that long phone call she had a few pages ago? Obviously, that flattened her battery. You need someone to take your heroine out for the afternoon to occupy her while she (and the readers) wait for a five o'clock gig? Wait on... those two men you mentioned do have an elder brother. You mentioned him six books ago, so you can dust him off, age him up to the appropriate vintage for the 2020 setting and voila! Better still, your heroine and he have something quite rare in common!
None of this is conscious invention. It just happens. Writer's serendipity. Where would we be without it?