OP I understand why you are careful on a daily basis, but when you are hosting, it is important to be generous and provide in excess for guests. It is also good to let DC have a treat during the holidays. Sorry but I think your Dsis is right.
Next time, explain that you like to be careful with your weight, but leave out a big bowl of fruit and some other healthy snacks and make sure they all know - your DC as well as their cousins - that they are welcome to help themselves if they are hungry between meals.
Bear in mind that DC who are going through growth spurts can eat vast amounts, and young teens, esp boys can need up to 3000 calories a day if they are growing a cm per month.
On an average day DS1, 13, who is skinny as a beanpole, can eat: a bagel with bacon, a banana and a glass of milk for breakfast; a huge bowl of soup, French bread and cheese, crisps and an apple at lunch, an adult portion of dinner (e.g. steak or chicken with potatoes or rice and 2-3 veg) and a yoghurt. In between he'll eat a couple of biscuits, slices of salami and some carrot sticks, and at supper a bowl of cereal. And he is rake thin from all his growing and nervous energy.