18 year old male cat

Hi Donna, it is hard to say why exactly your sister's cat is vomiting.  Aging cats can develop a variety of health problems, many of which a side effect is vomiting.  Kidney Disease is probably the most common health problem in older cats. 

The most important thing for a kitty of that age is to make sure he keeps eating and drinking well.  Hydration is critical to older cats, particularly if the kidneys are not functioning properly.  Canned food or baby food are great for helping to add moisture to his diet.  Sometimes switching around a diet can cause vomiting, so if you find a food that does agree with him, try to stick with it.  Keep plenty of fresh water available (in more than one location in the house) at all times to encourage the kitty to drink.

If you wanted to know for sure what could be causing the cat to vomit, you could bring him to the vet for a check up and blood work.  Something to consider though is the stress on a cat of 18 years of age by taking him to the vet and what would you do differently with a cat of that age if you did find out he has a serious illness.  Vets do usually carry prescription foods for geriatric cats with certain ailments.  For example there is a specific food for kidney disease, heart disease, diabetes and several others.  Again, that would require an office visit to see exactly what was going on with the kitty.

I have added some links to a few articles (click on the blue words) that you may find helpful.  Good luck and keep us posted on how kitty is doing and let us know if we can help answer any more questions.