I was surprised you were going to Dallas in August until I saw you were visiting family. Dallas isn't much of a tourist destination. It's not a bad place to live as far as the job opportunities/price of living ratio, but it is hot, sprawling, and ugly.

I usually leave Dallas whenever I have enough time off, so I'm not the best tour guide, but I'll second galen's suggestion of the Perot museum. It's new and it is a lot of fun. Dallas has a bunch of museums: a modern art museum, 6th floor museum (at the place where Lee Harvey Oswald shot the president), frontiers of flight museum, biblical art museum, dozens more I've never been to I'm sure.

If you want to see more of Texas history there is a bunch in Fort Worth, just a little west of Dallas. Their "Texas Heritage" is less overrun by millions of residents and new construction. The historic downtown has stockyards, the cowboy and cowgirl hall of fame museums, a bunch of old restaurants and bars, and rodeos almost every week. The cattle drive they do isn't worth seeing though, unless you have never seen a cow.

If your brother has some time off to go to a different city in Texas I would recommend San Antonio.

I accidentally posted this 6 times so I deleted the other 5. I'm going to blame it on flakey phone-tethered internet.