A high sed rate is usually a marker for inflammation or infection. When I was dxed with MS I had a sinus infection -- my sed rate was also high but it could have been due to the infection OR that the inflammation that comes when you're having an MS exacerbation. So, something is going on. But is a high sed rate unto itself is not a very good marker for MS -- especially if she has a sinus infection and her brain MRI is clear. The additional MRI you referred to is the c-spine MRI. MS lesions can also appear in the t-spine.
The vertigo and vision problems can be sx of MS but honestly, right now it could be a number of things. Lyme and MS often present much the same way and if your sister could have been exposed to Lyme, you should know that Lyme testing can be very unreliable. Like MS, it is a pretty complicated disease.
Take it one step at a time -- check in when you know more and we will try to help. There are some great people over on the Lyme board as well but try not to get ahead of yourselves -- it's hard but you don't have a ton of information at this point. Let us know what the other tests say.