I'm truly sorry to hear about what's going on in your life. Sometimes when it rains, it doesn't just pour, it floods. I know right now that you're not wanting to make things harder for your parents, but I will give you this piece of advice. I'm someone who let my parents pressure me into going into the career they wanted.
Don't do it. Not only will you continue to be unhappy, but if being a doctor is something you really don't want to do, then you might suffer through it today and tomorrow and maybe for a few years, but at some point you'll have enough and you may be resentful of it. I wasted 2 years of my life, my money, and my time going to college for something I didn't want and lord what a waste. I could be so much further along in what I really wanted to do if I'd have done that from the start instead of let pressure to get me.
You say that you hold in your thoughts and your feelings. This is one of those times when you really can't or you're gonna wind up spending your life living someone else's dream. If you're at all interested in medicine, then take a couple of classes of that and a couple of classes of aviation and see what you really like. But if your dream is to be a pilot and that's what you really want, go for it.
Explain gently to your parents that you know that they want what's best for you and you appreciate their input very much and you know that it's been a rough time for everyone, you included (tell them how you've felt - you can't bottle that up), but that if you don't at least try to follow a dream you are going to spend your whole life wondering "what if".
That you are standing at a crossroads where you have to make a decision that you feel comfortable living with and that you know that what they truly want for you, what they've worked so hard for all these years, is for you to be happy and be you. If this is who you are, then tell them and ask them to love and support you no matter what happens.
At the very worst... medical school can wait one more year. You'll still be smart in a year from now. Most of all, look on the bright side. You're about to go to college, begin your life, maybe live in a dorm or an apartment, meet new friends, start to really live again. It also sounds like your little sister is on the road to recovery. I know it doesn't seem like it with all she's had to suffer, but it's something to be thankful for. Maybe you can find a way to inspire her? To show her that nothing is beyond her reach? Or maybe you can let her inspire you. She sounds like a fighter.
Also, find a therapist you like, interview a couple of them until you find one that clicks with you. And have someone to talk to. You can't bottle all of your feelings up, it'll eat away at you until it comes out at the most inopportune and terrible moment. You definitely don't want to be giving a presentation in class and suddenly burst into tears. The human body has to grieve, has to express pain. It's healthy. To suppress it is very unhealthy and it will bite you in one way or another. Some colleges offer free therapy to their students if your family can't afford it right now. Look into it. I know the budget is tight, but don't put it off too long.
I really wish the best for you and your family in this difficult time and that things start to really turn around for you all. Take care of you.
