Question #1 (because I am very forgettable) are they spayed?

Thought #1....Could Kimi have a urinary tract problem? If it is out of character for her to pee inappropriately, this is a good indicator of a problem.

Thought #2.....could something be preventing them from getting to the basement to the litter boxes? (any chance they had a scare down there recently, workman, loud noise, etc?)

Question #2.....how many litter boxes are available?

The plan of action I would take.
-rule out medical problems in Kimi.
-Attempt to retrain Amaya to use the box (usually done by confining them to a very small area until she uses the box exclusively and then allowing more area, but not the whole house yet)
-And I oersonally would make sure there were litter boxes available on ALL floors of the home (just my personal opinion that going down 2 or 3 flights of stairs to get to the bathroom is too much to expect from a cat.)