Hi there -- I think I may actually have some usable experience with this!!
In my case I'm the one who needs to keep food and calories restricted so as not to balloon, my 7 year old (as of today!) is a little chubby and definitely benefits from 3 healthy meals with lots of vegetables and lean protein and some whole grains, some healthy fat and a fruit dessert, while my husband is a body builder whose tendency is to be very thin and who is bucking nature the whole way as he loads on muscle. I would not recommend that anyone eat the way we do, because it has a lot of problems, but the basic concept might help. Every morning, Steve makes his own breakfast (3 eggs, 3 egg whites and oatmeal, and sometimes bacon) while I make oatmeal for my daughter and a particular breakfast for me that doesn't matter here because you are trying to gain weight. I make my own lunch, my daughter gets lunch at school, and Steve eats two lunches, one around 11 and one around 2-3 -- both very clean -- both, vegetables, whole grains and not a lot of fat, but two whole lunches. Once a week we try to meet to have lunch and so he eats one of his lunches with me around 11:30. Then at dinner we often eat together and sometimes we even the same thing -- protein, vegetables, whole grain pasta or brown rice, maybe some tomato sauce, or sometimes we do what we do for breakfast with Steve making his own dinner, and me making something like Amy's spinach and pizza rolls of some vegies for my daughter and tofu and vegetables over a salad for me. Before bedtime Steve eats another meal -- it used to be a sandwich, but now its pure protein -- turkey and cheese.
The basic idea that I think might be usable here is that both nutritionally and socially, you will be happier if you don't take in all your calories when the other two members of your family do. You can eat meals with them, and then just eat two additional meals to get your food intake up to what is right for you. A great source of information for this stuff is Bodybuilder.com --particularly the "over 40" section. I have never gone on myself, but my husband lives there and there is a lot of good information about nutrition that circulates there -- one thing I learned from them, through him, is that there are three basic body types -- one type struggles to keep weight on, one is neutral, and one must work to keep weight off. It helped to realize that my husband and I have different body types and that to be healthy we really do need to think about food in very different ways, but it is also great to know that "good food" is pretty much "good food" -- that is, a high ratio of vegetables and fruit, lean protein and whole grains without too much fat (but definitely not cutting out all fat, which is dangerous), and choosing healthier fats like olive oil, avocado, butter and nuts is good for everyone in the family, but the amounts need to be radically different based on body type and exercise levels. If eating so often strikes terror into your heart, there are of course some good supplements out there -- milk shake type things, that could easily replace one of the extra meals, and maybe both if you're desperate (but check all this out with your doctor or at least on a site like Bodybuilder. com where folks are dealing with the complexities of healthy weight gain and nutrition constantly.)
hope this helps!
M.