Before concluding that the issue is behavioral and nothing else, I wouldpursue an evaluation by an Occupational Therapist. It may be that she dispays a Sensory Integration Disorder and that, for her, the 'simple' feeling of clothing can be quite uncomfortable.
Maybe her sister's clothes, assuming her sister is older, are larger in size and do not touch her skin as much feeling more confortable to her. Listen to the Doctor, there is such a thing as Sensory Sensitivity in some kids that causes them to feel very unconfortable in clothing or ceretain touches or noises because their senses are not working well together. Go see an OT specialist as the doctor suggest and make sure that she does not have such a disability. The good news is that with growth and maturity her senses will mature and be able to handle this issue, most of all if you get the help of an OT specialist. You can google sensory integration disability and see if you can find some articles to read. She probably does not feel very good in the clothes and it is not her fault.
It is posssible that her behavior is primarily willful/oppositional, but it's important to cover the bases and not miss something that might be occurring. From the behavioral perspective you can rely on the approach detailed in Lynn Clark's book SOS Help for Parents (see www.sosprograms.com)
I still don't understand why she will be fine wearing her sisters clothes but won't wear her own..she makes excuses not to put the clothes on but if she is really pushed and she learns she can't go somewhere because she doesn't have on her clothes and we start to leave she will put them on..but still throw a fit and take clothes off in the car
She even complains everything itches, hurts, tickles..Is it just an excuse not to put anything on or is this truely a behavioral issue and how would you handle this? she screams for hours if you are trying to get her dressed