I found the info we need about how we gain XP:
http://ddowiki.com/page/Experience_points
These are the most important penalties:
Level of quest vs level of highest character in quest: (party)
Compare the quest's level to the level of the highest-level character to have entered the quest (even if they later left). If that character's level is below, equal to, or one level above the quest's level, there is no adjustment. If it's +2, there's a -10% penalty. +3 = -25%, +4 = -50%, +5 = -75%, +6 = -99%, and at +7 or higher there is no XP regardless of bonuses. Casual difficulty subtracts 1 from the quest's level (if possible) for purposes of this adjustment, Hard difficulty adds 1, and Elite adds 2.
Power-leveling penalty: (individual)
If you are exactly 4 levels below the highest-level character in the party, you receive a -50% penalty. If you are 5 levels below, you receive a -75% penalty. If you are 6 levels below, you receive a -99% penalty. If you are 7 or more levels below, you receive no XP regardless of bonuses.
The powerlevel penalty explains why Khethermyl got less XP than me. I was level 8 and she level 4. That means she will get a 50% XP penalty in addition to the penalty for playing on elite first part (effective quest level 5 vs highest party member 8 = -25% and hard (effective level 4 vs highest party member level ) -50%)
Since this was first time playing at the level for both of us it meant we got 25% bonus to XP.
This means T got only 50% XP when we played on elite and 25% XP when we played on hard. I would get 100% XP when playing on elite and 75% XP when playing on hard. In addition I have a 5% XP bonus item which adds further.
Since we only did part 1-3 on elite it means that T got very little XP for the last part which had the highest absolute XP rewards. That explains why she only got 2k XP and I over 10k.