Pawn on a fight that isn't even mine.

Check out www.etiquettehell.com. lots of help there. If you are truly safe and they are just being a**es it is truly none of their business when your mother gets home from work, sleeps, goes out or even breathes. I am guessing from your post this is your dad's mother and sister? Sounds like they are trying to put themselves in you'd parents marriage. Which is only for your parents. Do you have caller I.d? If so there is no reason you have to pick up the phone. Let it ring/go to voicemail. Your parents should have already shut this down long ago and protected you. You are right that she shouldn't have called you at 16 with the suicide bit. Someone should ha e called 911 (on several people who have pulled that to disrupt their children marriages/just want attention and know this will get it have stopped after people have called their bluff and done this.) And then told your gma how inappropriate this is.
It is inappropriate for them to be doing what they are doing to you. Sounds like your mom could use www.motherinlawstories.com you can too as there is a lot of people dealing with in-laws doing this exact stuff. A lot of good advice there how to deal with it.
Tearing your mother down on front of you shows they don't care about you or your feelings. And so your dad goes to the store alone? He is a grown man he can do that. And if they thought your mom couldn't handle managing the house the proper thing to do would be to offer help not tear the person further down.
You do NOT have to tell them your family's business. You do NOT have to go to lunch with them, just tell them "I am afraid that isn't possible" lather rinse repeat if they ask why. You don't have to 'jade' either -justify, argue, defend, explain. And to give you grief for going to lunch with your own mother is more bs!
Have you tried telling your parents how all this is affecting you? I see where you say you have trouble -trywith showing them your post. Copy and paste it in notepad, explain you have trouble with saying this kind of thing and ask them to read it. If they still don't put their foot down you will have to learn to develop a spine and skin that will help protect yourself from their poison.
Good luck! Feel free to email me if you just need an ear to listen.