I know you're at the beginning of your weight loss journey but there are going to be many many times that you'll have to face these dilemmas. Eating chicken and potatoes and corn is really not going to put you at a great risk here, especially if you plan on proportioning your plate now.
Try to eat white meat, and leave the skin off. Do you have a scale? Try to keep your portion 4-5ounces which is little more than half a breast. Eat the noodles OR the mashed potatoes, you can't have 2 starches, say it goes against your religion

and believe me, it's not rude to pass on one. Everything else just portion it out, 1/2 cup, whole cup, whatever calories you're willing to fit in. Ask your Grandma if there's butter in it in a way that's not persnickitty "Grandma, these potatoes are soooo good, how did you make them anyway, I bet there's loads of butter in it!"
There's always a way to get around family meals and temptations without alienating the people around us. While you're at dinner with your Grandparents you may want to have a conversation with them about your attempts at better nutrition, I'm sure they will be very supportive.