Damsel is right, having an eating disorder is not a decision you make, I never knew I was anorexic when I lost weight, I just thought this was what I looked like and if it took eating very little then that didn't bother me.
Eating disorders are so so complex and food is only the symtom of underlining issues. It does seem that you may have somethings you need to work on but you are young and have your whole life ahead of you. It would be so great if you could deal with all the stuff you need to deal with now because you will be free of guilt and all the other destructive feelings you have.
Anorexia, bulimia, over eating, they are all the same distress and so where I go to therapy we do not call it eating disorder, we call it eating distress. Eating distress represents the thoughts and feelings behind the behaviour, while eating disorder is just labelling the behaviours you use in order to deal with your emotional distress.
You are unable to " fix" your parents marraige although I know it must be difficult to be in a house which is so unhappy, it is so important that you look after yourself and do whats best for you and not for ayone else.
I know that your gran would not want you to become ill in order to be with her while she is in hosiptal, just the fact that you care so much for her and want to help her is enough.
You seem like such a caring person, who is very sensitive and gentle, they are great trates to have but you have to use them for your own good as well.
Your parents are going through a though time and I know you are affected by that but you are not controlled by what your parents are going through. You are your own person and you deserve to be free of all the things you are feeling which lead you to want to not eat, you are so much more then that.
I have had an ed for 5 years now and I have suffered enough, I do not want anyone to suffer needlessly because in the end you are going to have to deal with how you feel and think sooner or later, I just hope that you are one of the lucky one's who has not asked for help too late.
When you suffer from ed, weight doesn't become the only thing you want, you can become very depressed and its not good at all believe me. I also want to mention the medical side to it, there can be so many medical complactions that you might not be able to do sports anymore, your organs could be badly affected, there are just so many things that could happen and death is one of them.
This is not something to mess around with, if you can ask your parents to let you see a therapist, I think it could really help you.
You do deserve to be happy, to be free and to enjoy life and who you are.
I hope you are doing well and if you ever have any questions or just want to talk, i'm here.
Take care!