We moved to a GCC recently, my wife and step-daughter are of different nationality to me. We had a nightmare obtaining a residence visa for my step-daughter despite being told by the company before joining it shouldn't be a problem, and having spent hundreds if not thousands getting all our legalisations and translations in order. The visa application was declined 3 times, the company didn't help at all, my step-daughter was unable to attend school, and they were ready to go home. We discovered at the 11th hour that a large brown envelope with the best part of that month's salary was required. Dodgy meetings in car-parks and forged signatures followed, but we got the visa.
Residence visas are issued for step-children officially - allegedly - but no one from the immigration service, embassy or company could tell us the rationale on whose application gets accepted and whose doesn't. Good luck if you're going to try it!