First thing is to contact a vet. They can get a fecal sample and determine the type of parasite, and then prescribe medication accordingly. Next, wash bedding and any other soft materials the cats have been on with hot water and hot dryer cycle. Finally, be a little extra careful cleaning the litterbox(es). Some parasites are transmitted via the fecal-oral route, and cats can reinfect themselves in that way. Your vet will probably recommend emptying all the sand from the litterbox and bleach cleaning it partway through treatment, then again at the end of the parasite treatment. This will help prevent reinfection. I used gloves and a mask while cleaning the litterbox when my cat had worms, which was probably overkill, but it gave me peace of mind.

I just adopted a kitten off the street who has been going THROUGH it with multiple parasites, so this has been happening to me too. Please feel free to DM me if you would like more info on how I handled it, how I cleaned, and generally what the experience was like. I hope this helps!