You could check Bioware Technical Help too. the problem has been addressed here:
http://swforums.bioware.com/viewtopic.html?topic=290227&forum=80)
and here:
http://swforums.bioware.com/viewtopic.html?topic=327844&forum=80&highlight=sand)
(try a search for sand people in the technical forum)
Extract:
You may also find that currently if you do not wear the sand people uniforms, the sand people do not go hostile towards you. This is because the variable that says "I'm in disguise" is still set. Putting on the sand people uniforms will reverse this so it will say "I'm not in disguise" This makes the sand people hostile. So there are two ways around this.
1) Take off the sand people disguises before entering the second dune area (the area outside the sand person enclave also known as tat_m18ab). This will mean you can complete the quest but everything will be confusing as the states are reversed.
2) This is my prefered work around. Take off the sand people disguises and enter the area outside the sand people enclave. Then use the "Return to Ebon Hawk" function to return to Anchorhead. This should set the variables correctly once more. Now return to the dune sea by walking as opposed to transiting back. You should be able to use the sand person uniforms as normal.