Has this behavior started BEFORE the addition of the new people? If not then YES, it could be the reason.

When the people moved in, did things change? For example:
-litter boxes were moved
-feeding times were changed
-noise level increased considerably
-Ninja was closed out of rooms that she was formerly allowed to go into.

Anything could upset a cat even if we think it is a minor change. If change stressed her, then she could be acting out toward the other kitties.

This would still suggest a vet visit because she might be able to take a medicine that releases stress.

But if you DO think of a change in her routine, then getting it back to normal might be a big help.