User blog comment:Fandom Freak002/My Thoughts/Opinions on Dragon Slayers/@comment-24277053-20141229193800/@comment-25405856-20141230010039

2 possibilities why she is not affected by motion sickness:

Something along the lines of a 'permanant troia'  (unlikely)

Seeing as Gajeel's motion sickness only developed around the time of the Grand Magic Games, Wendy's will probably take a while to develope, too.