Kin selection applies to calculations of parental investment and cases where there is parent-offspring conflict. Since all multicellular creatures have a parent-offspring asymmetry and provide some kind of resource bolus to their offspring, that means that kin selection applies to all multicellular organisms.
In other organisms, kin selection is also often relevant. Many microscopic organisms exhibit limited dispersal - and so are surrounded by their kin. If your kin are nearby, you should typically curb your pollution, share your food, and commit suicide if you are compromised by pathogens. Many parasites spend much of their time inside their hosts - where they are often surrounded by their kin.
Kin selection also applies to cultural variation - in the form of cultural kin selection - where it is similarly ubiquitous.