Do cats feel depression or a loss?

About 4 years ago I lost my grandma she was sick alot in and out of the hospital. When she passed I away I adopted her cats they are brother and sister.

Within 6 months after she passed away my brother cat got really sick he wasn't eating he was getting dehydrated. There were atleast 2 times I almost lost him. Can a cat be so depressed it makes him sick. He was so used to her coming back from the hospital when she didn't come back anymore could he have made himself sick. He was very attached to her! He has IBD and a fatty liver disease and has been threw alot. They gave him the steroid prednisolone(something like that) to get him to eat. It worked but it also raised his sugar level really high now he has diabetes. He has to eat specific food and take insulin along with other medicines. But he is well worth it. I was told by his doctor that taking the steroids shortened his life is this true if so how much? How does diabetes affect him in the long run?

My sister cat is fine. They are liter brother and sister. Do things just affect them differently?

Cats, like people, react differently to losses. I'm sure the female was mourning too, she just didn't show it as much. Long term steroid use can shorten the life span, usually by hurting organs like the liver or kidneys. I have given steroids for cancer for over a year with amazing results, he was like a kitten again, so it was well worth it. Cats can live a long time with proper help for diabetes, I knew one that had injections for 10 years! Just make sure you have him tested regularly, the high blood sugar may be temporary from stress or illness. I want to thank you for taking in these two babies, they are so lucky to have you. Bless you for your kindness to them. All the luck!

Welcome to TCS 

Here's an article about grief and loss in cats - 

Do Cats Mourn?

And a couple about diabetes - 

Introduction To Feline Diabetes

A Feline Diabetes Story

As for steroids, a lot depends on dosage and length of treatment. Steroids are a double-edged sword. They can perk up a cat and alleviate symptoms but they can have adverse reactions, effecting the liver and kidneys. It's hard to know how much the steroids affected the cat. The only one who may be able to assess things in that level of detail is the vet who can review the cat's course of treatment as well as his current condition.