Sorry to hear things have been rough for you. Maybe I can offer some words of comfort.
"One thing after another" - Life always feels like that. It could help you to develop different thinking styles to reduce stress, phrases to tell yourself, or activities to partake in which have a calm association. Because I can assure you now, time heals all, and you will be okay.
Nobody likes to see their pets die. But alas, it is the circle of life--- Everything that lives, must die. As such, the best thing you can do is ensure that they had a full, loving life. From what I know of you, I'm sure these animals were treated well. Even if their time was up, the time they had with you was spent in the best way possible. You will have fond memories of them for your entire life.
As for stents, I cannot find much information on them, but I believe your grandfather will be able to survive years to come so long as he keeps up with his health and medications. The same for your grandmother--- I have known people in their 50's who have had minor strokes with no lasting effects other than needing to make lifestyle changes. I'm sad to hear she is having so many health scares. Cancer will get most of us eventually, and thankfully hers was caught and it will grow slowly. Knowing it's there can be scary, but perhaps it will have no impact on her quality of living nor her length of life after all? How is it looking lately?
Death from epilepsy is rare and can only be caused by breathing or heart problems. Your sister is young so I would be surprised if she was really clinging for life. If she is someone prone to seizures, and by the sounds of it she may get them when she gets too excited, then she will need medication to manage these symptoms and will otherwise lead a mostly normal life (I believe). It is unfortunate so many members of your family are suffering from health problems all around the same time, but I think it will sort itself out so long as they all look after themselves and listen to their doctors. I know how you feel as there have also been some issues and losses in my family over the past couple years. You're not alone.
Take all of this in stride and don't forget to focus on your own mental health. If there is nothing you can do about a situation, try to minimize your stress by not thinking too much about it, or remind yourself it will be okay. It will. There is no shame in taking anti-depressants, and by all mean feeling depressed right now is entirely valid. If your friends don't believe you, it's just because they're possibly too young and ignorant to the situation right now to fully understand you. It's a shame because I know you would like their support right now. If you need a place to vent, we're here for you. If you dislike venting, it may be good for you to create a journal to write in privately.