If you have set the precedent for treating to dinner, as most guests do as a thank you for the hospitality and them giving you a free bed and board, then you either invite the household adults or  whole family or invite no one. The advice given, I think, is just plain mean and would end your free bed and board at my house in a hurry with a gentle hotel ad sent to you next visit. If this is deeply offending you so much, then buy ingredients and cook a nice  dinner for the hosts or get some really nice take out. Basically, family or not, you are in essence doing as you accused your nephew of doing, freeloading off them (I assume you are not compensating them for your bed or food). So why get so petty about it? Try giving your nephew a hug, telling him you love him! Share a glass of wine with him. Be the bigger person! Embrace the love and joy of the upcoming season with a new start!!