I am sorry to hear you are getting pressured by your family to lose weight. I agree that you should sit down with your grandmother and explain to her that bribing and nagging you to lose weight isn't going to make it suddenly happen, and is in fact detrimental to your efforts. You should be making changes for yourself, not for some old bat with nothing better to do than badger her grandchild.
It may be difficult for you to get the message across to her, but try your best to ignore her and don't let it get to you if she continues to harass you about it. Talk to a friend, go for a walk, or get on Spark when you feel like you need support! Walk out of the room as soon as she opens her mouth if need be; you leaving isn't rude, but her haranguing you is.
Stay strong and come here for support when you need it!