Hi Katherine,
How about having several tasty salads for them to choose from? This site has some very nice bean salads that are filling and satisfying, and you could add a vegetable salad or even just some raw veggies with a nice hummus or dip. Here's a recipe for a
quinoa and chickpea salad that I've been eating a lot this summer. The recipe calls for seasoned rice vinegar, but I didn't want to use that because the seasoned rice vinegar available here has sugar and salt added to it, so I've been substituting Acid League's Meyer Lemon and Honey vinegar, and it makes a very tasty dressing.
As for dessert, what could be better than fresh fruit at this time of year? Whatever looks best in the store is what I'd go with. Of course, if you want to be a bit fancier, you could go with nice cream or one of the fruity desserts on this site - I haven't tried many of them, but I think they're probably all good.
Hope you have a good visit with your brother and his wife!