Some cats just eat too much or too quickly and make themselves sick, but other times it can be a sign of an underlying medical problem... especially if it happens a lot. When a cat who is very heavy starts to lose weight she can develop serious health problems like hepatic lipidosis (fatty liver disease). I would seriously think about getting her to the vet to have her checked up, just to be on the safe side.
Actually, most of the time when a cat stops using a litter box there is a medical reason. One of my cats started this recently, so I took him to the vet. I was concerned, because a cat's normal instinct is to bury its waste when it goes to the bathroom so predators don't fint it. It turned out he had a urinary tract infection, and he associated the pain from urinating with being in the litter box. The vet gave him antibiotics and he got better and went right back to using the litter box normally.
Have you had the cats very long? It can be normal for them to hiss and act up like this if they are under a lot of stress, which can be caused by all sorts of things, like a new person coming around, or moving to a new place.
Do you just have one litter box? If the male is marking, adding a second litter box might help. He could be "claiming" that box as his territory, so if she had another litter box to use he might stop.
I feed my cats a canned food called Eagle Pack. After studying pet nutrition a lot, I have learned that a quality canned food is a lot healthier for cats than dry food. Here is a list of some quality canned foods... the Eagle pack does have some grains in it, so it is less expensive and still a nutritious solution.
http://www.catinfo.org/commercialcanned ... NNED_FOODS
(The information on why canned food is better than dry is here, if you are interested: https://www.catforum.com/viewtopic.php?t=9107