In any match as isis it'll be a good idea to build a monument at each of your bases, and in any deathmatch, especially in your case, you may want to get some mu's.
As isis in classic you can get sphinx, which is very powerful when fully upgraded, and it will be automatically.
Then heroic you can get pestuchos which is a great support unit and rocs are a good way to avoid walls or drop siege on building sites.
Mythic you can get mummies which some people think are weak but they are actually VERY strong due to the fact that they can instantly kill enemy units and for a bonus they become minions which are strong units as well.
Phoenix's would be great to because they are flying, have a splash attack(can hit multiple units with one attack), and are untouchable by spearmen. These can be hard to stop when massed because you need to make lots of priests or chariots which will weaken your opponent's main army.
Keep in mind im not saying you have to go for these gods. For example classical i'd probably go Anubis for the fact that his upgrades benefit you much more in a dm, but myth units can always help.
If he tries to copy your moves with mu's you can definitely have an advantage by going Nephthys because she makes priests cheaper, stronger, heal faster, i think move faster and get more range but i'm not sure.
I hope this information helps but make sure you don't mass myth units to hard, otherwise he can just get purely priests and take them out easy, while only focusing on gold.
GOOD LUCK!
EDIT:
Something I just thought of that also may help.
In a deathmatch against your brother it could be smart to go as Set because you can get very strong slingers with extra attack and health that train faster, and you can get electrum bullets, and slings of the sun which increases your attack vs. infantry.
And by going Sekhmet ou can get bone bow which adds +4 ranger to chariots which allows them to attack from farther back and gives them more range than tc's.
You also get Scarab's which are siege that aren't easily stopped by spearmen and his spearmen will focus on them while your slings fire away, and you get siege upgrades that help catapaults and rams.
Lastly, Citadel which gives you more population to help if your brother has a tc advantage.
Good Luck again
[This message has been edited by Nooblet (edited 12-02-2006 @ 03:06 PM).]