Hi phab5ive.
Congratulations on getting a chance at the interview. You might want to also check out the pprune forum thread etihad-world-wide-cadet-scheme to see if anyone else got an invite.
I am not an insider here so can't specifically say what goes on with the cadet scheme but speaking generally they would be looking for cadets with 'the right stuff '. That would be personality, motivation, aptitude, education standards, CRM type skills, multi-cultural tolerances, communications skills and hand-eye type coordination skills in line with aircraft manipulation. After all they would be making a large investment in training the cadet.
Basically I would expect the interview process would follow the general pilot assessment interviews such as an English test, Technical test (maths, science and general knowledge), Psychological testing, Group exercise, personal interviews and extensive medical.
If they did have a sim session I would expect it would be general co-ordination exercises (look-do and follow verbal instructions). Treat this as fun as after all they can't expect a new cadet to have advanced flying skills to handle V1 cuts and 1E Raw Data ILS!!
Overall be yourself and be professional. Come prepared. Read up about Etihad general info, the places they fly and some UAE general info.
Have fun and good luck.