User blog comment:FairyRaven/Exceeds and Earth Land Counterparts/@comment-4829046-20120820134427/@comment-904888-20120821010244

Wendy and Cobra are easy to explain:

1. Wendy's healing magic would probably prevent her from having the motion sickness, and back then, she might not be as advanced as Natsu, so she doesn't (at least didn't) suffer it

2. Cobra wasn't nearly as advanced as Natsu, as evidenced by the fact that when he decided to use his DS powers, he had to go into DF mode to use it (same with Laxus, initially). This strongly suggests that his inherent DS abilities are very weak or pratically non-existant, so again, he wouldn't suffer Motion Sickness due to his lack of advancedness.

I am not saying it's a strong plot device ( in fact I find the motion sickness concept somewhat ridiculous, even for the comedic value), but it fits what was there already.