(snip)
> They could find another feral cat colony caretaker.
It's possible, but not likely. No one should hand over the maintenance
of a feral colony to someone they don't know without a plan to check on
the cats on a regular basis. It takes dedication and money to feed,
shelter and vet a colony of feral cats, 365 days a year, year after
year. Most people who are inclined to do so are already maintaining a
number of cats and might not be willing or able to take on more.
> Call cat rescue and feral cat groups for help.
It's unreasonable to expect feral cat groups to have rosters of
volunteers waiting for the chance to feed more cats. Feral cat rescue
groups are overwhelmed with abandoned, starving, sick and unaltered
colonies. People are lucky if they can get assistance with TNR and
kitten socialization.
Presumably, the poster's sister and brother would not move away and
leave their housecats behind. Responsibility-wise, feral cats are no
different.
> Or, they could relocate their colony to their new place.
Yes, this is the best solution for everyone.
> They probably won't be able to catch them all.
What on earth makes you say something like this? As long as no one else
feeds the cats and there is time enough for a little trap conditioning,
it's a slam dunk.