Thats a good point. I just have this sneaking suspicion that she will be a part of the main cast. I would love for that to be true. However theres a chance she could be a recurring characterHmmm, I have this nagging feeling that there's a lot of stuff the anime staff is privy to that we aren't yet. I'm sure this is usually the case, but showing off Viola is an interesting decision no matter how you slice it. If I had to venture a guess, they probably intend for Viola to appear in more than one or two episodes. IIRC, back in the Sinnoh days, most of the gym leaders were around for four-ish episodes and, I'm sure like some of them, there are probably gym leaders in X and Y that will play a part in whatever the plot of the games are. That trailer earlier really suggests that they intend for something notable to happen pretty quickly, so I wouldn't really be surprised if we see Viola involved in said events.
Just because she is Alexa's sister does not mean she will be a recurring character. and if that is the case thats just poor writing
Do we know what gym number Viola's is??smash says that clemont will be the first person ash battles in kalos... so, similar to dento, ash would battle him for his first badge. viola will probably reoccur with her sister. one may recall an older trailer with bonnie, clemont and alexa watching ash battling someone, probably viola. but, we shall see.