Good manners dictate that the one preparing the meals (doing the shopping, fixing the food, and covering the cost) is the one who sets the time and menu for meals. A good houseguest bends and is gracious. I, for example, am a vegetarian. I would never in a million years expect my hosts to cater to my special dietary requirements. I tell them I'm a vegetarian in advance and then I'm grateful for whatever they serve and whatever I can eat.
While it's especially nice when my host has a vegetarian entree for me, I certainly don't expect one. My 80-year-old mother and 78-year-old mother-in-law love me dearly, but they don't accommodate what they consider to be my “crazy” eating habits. They're used to serving meat and potatoes for dinner. Therefore, I stop at the farmer's market before my visit and pick up fruits and vegetables for everyone to enjoy. A good house guest does her best to be low-maintenance.
With that being said, if you're staying with your brother's family and they eat later than yours, don't hesitate to discuss this with them before your visit. Explain your concerns and try to reach a compromise. If you have little kids who are accustomed to eating at 5, they may not be able to make it until 7 without getting crabby. If you're on vacation and want to wake up early for sight-seeing, you may want to eat earlier as well. Communication is key to preventing misunderstandings, frustration, and hurt feelings.
Most hosts will bend over backward to make their guests feel at home. The only exception is when they feel taken advantage of and disrespected. A good houseguest acknowledges and appreciates their efforts in words and deeds.
I've had people in their early twenties stay with us and, because of their limited life experiences, they're often unaware of all that's involved in preparing a home for guests: making up the beds, scrubbing the toilets, buying the groceries, planning the meals, etc. It's that lack of awareness that makes hosts feel overburdened and used. If you pitch in with the meals—cooking, ordering pizza, or taking your hosts out to a restaurant—you'll make a good impression.