A.K.
B., I'm so sorry about your troubles. You're young and it's difficult to have the weight of the world on your shoulders. Your family seems like it's been dysfunctional for generations. I understand that you really want to help your grandmother. Unfortunately, I don't think that you are going to be able to change either of them at all. It's been dysfunctional for many, many years, and their old patterns are very ingrained.
I recommend that you seek professional help for yourself whether from a licensed independent social worker (LICSW), psychologist or therapist. Speaking from experience, I think you will need help figuring out how you're going to handle this burden and at the same time not feel like you're responsible for fixing it. Your physician or ob/gyn should be able to give you a referral. If you're in college, maybe there is help offered there.
I also recommend that you seek out and attend Al-Anon and/or ACOA (Adult Children of Alcoholics) groups in your area. ("Al-Anon is a worldwide organization that offers a self-help recovery program for families and friends of alcoholics whether or not the alcoholic seeks help or even recognizes the existence of a drinking problem.") (ACOA is a program for women and men who grew up in alcoholic or otherwise dysfunctional homes.) If one group isn't the right fit for you, check out others until you find one that's suitable. There are lots of them around, usually held in churches, hospital conference rooms, etc. The best part is that they're FREE. You may not connect with everything that's said the first night. Some of it may hit home immediately and other stuff may take a while to reveal itself.
Perhaps your grandmother can find a professional to speak with as well. I'd also encourage her to come to the Al-Anon or ACOA meetings with you. You may even want to check out some of the enlightening self-help books at your local library.
Best wishes, B.. Keep being supportive of your grandmother but try not to get too emotionally drained from all of this. You are not responsible in any way for these problems, you are not responsible for fixing them, you are not responsible for speaking up to your grandfather, you are not responsible for fixing their relationship, you are not responsible for helping your grandmother get stronger, you are not responsible for your grandmother's happiness. She's an adult, and she does have choices she can make, even thought they're very difficult. These are life-long issues that would best be handled by a professional.
I apologize if I've gone off on a tangent. I truly hope this helps. Wishing you peace.