So sorry to hear about your grandmother.
I too used to shy away from not only family gatherings, but people in general. I was 360 pounds, had nothing "nice" to wear, and felt constantly judged as the fat outcast. What I eventually learned was that I have to love myself before anything else, that it doesn't really matter what others think. Make sure that you want to do this is for you, not to impress relatives/friends/etc.
If you want to get serious about getting healthier, start writing things down. And no, I'm not talking about a tracking your food (although I'd highly recommend that as well as it definitely helps), but writing down your reasons. What are your goals and why? Even if it sounds silly, it's a way to truly explore the things you want from life, and an important stepping stone to making things better.
Once you have your reasons in black and white, take the next step and brainstorm about positive ways to make it happen. One of the first things I did was browse through the nutrition info of my most frequented restaurants and made lists of what I could fit into my new plan: those lists stayed in my purse so I always had them to refer to.
Also, find ways to set yourself up for success! Keep your kitchen stocked with healthy options. For example, I always keep a big bowl of prepared salad to ensure that I can have fresh veggies and fruit with every meal. Focus on how your choices will shape your future and make ones that you can be proud of. What's going fuel your body properly and give you more energy, a greasy fast food meal or something home cooked with lots of veggies? Do you want to be weighed down (both physically and emotionally) by your choices? Set yourself free. Learn to work with your body to allow it to heal.
You can do this.
Take care of yourself and make Nonna proud.