One of my cats died, how its its siblings react?

Tragiclly I lost one of my cats today, he suffered a heart attack this morning and died next to me. He was not even 2 years old yet. After returning from the vet his two siblings, a male and a female seemed a bit scared, I think they could sense something was wrong and were afraid to come near us at first. They seem ok at the moment, but will they realize their brother is not coming back? And if so what can we do to make sure it is not stressful for them. We are going to take them to the vet next week for a check up (a few months earlier than thier yearly scheduled) because the emergency vet we went to said the heart problem could have been hereditary, but he was over weight too. thank you for you answers