I just came from Luxor yesterday having spent 3 days there, and we were very lucky with the weather - i.e. it didn't get much above 95!!! Sorry, but the weather in that particular part of the world is best described as "oven-like" from May until late September.
However, it IS manageable, you just have to pace yourselves. Most of the tourist sites open very early in the morning (6 or 7am - e.g. the Valley of the Kings) so you can safely visit them before the worst of the heat kicks in. A few places are open again in the evening, so it is possible to spread out your visits and rest in between.
My recommendation is to start as early as possible each day (6am if you can manage to get up that early!), tour till 11 or 12 maximum then retire to the air conditioned haven of your hotel for a cooling dip in the pool, lunch and a siesta. You can then go out in the evening and visit among other things:
... Luxor temple - open till 10pm and breathtaking at night
... the Luxor museum and Mummification Museum - both excellent
... the Sound & Light show at Karnak
... take a caleche (horse drawn carriage) along the corniche
... or stroll through the souk (bazaar).
The most important advice I can give you is:
1. Pace yourselves - don't try and cram too much in, or you'll end up exhausted and won't enjoy anything!
2. Avoid touring in the hottest part of the day - noon till 5pm.
3. Drink plenty of water to avoid getting dehydrated - sounds obvious I know, but honestly the air is so dry down there that you can be sweating profusely and losing crucial salts and not even realise it! You should drink 3-4 large bottles of mineral water per day. IMPORTANT!!!
4. Wear heavy duty sun screen at all times, and some kind of head covering.
5. Take some kind of fan with you - the cheap chinese folding kind, a piece of cardboard, ANYTHING will do, as the heat inside some of the tombs can be absolutely stifling and any slight air movement makes the world of difference, believe me.
I hope this advice hasn't discouraged you too much! It WILL be terribly hot, there's no avoiding it I'm afraid, but if you go in knowing what to expect and take the above precautions, you can still enjoy your time in Luxor! :-)