I wouldn't worry too much about your sister undergoing tests to check for MS, as much as there's still no known specific cause for it, new treatments are being found all the time for those newly diagnosed with it.
I was diagnosed (dx) with MS in 1994 at the age of 26. At that time, I'd only ever been aware of 2 people who had it, one was a retired doctor for whom my mother was a home help, and my friend's mum's partner had been dx 18 months before me.
I didn't know anything about the different types/stages of the condition back then, only that both of the people I knew with it were wheelchair-bound.
Needless to say, I was terrified.
I have done a lot of reading over the years, and did actually discover that my mother's boss had Primary Progressive MS (PPMS), whereas the other guy and myself had Relapsing Remitting MS (RRMS), and that he had 'given in' to the condition and put himself (unnecessarily) into the wheelchair within a year of dx, which all gave me a little more hope than I'd had on the day of my dx.
I've also discussed my medical history with my specialist team, and it's apparent that I'd been suffering with numerous symptoms of the condition since I was around 9 years old.
I'm now 55, I'm still on my feet, although I can't walk very far, and I need a walking stick these days. I do think my stubborn outlook has helped to keep me going, as well as living alone makes me aware that if I don't do things for myself, there's no-one here to help me, so I keep plodding along, taking each day as it comes.
I've had a lot of issues with the condition, had various treatments for numerous symptoms, but NO treatment that's labelled specifically as being for MS. This is more than likely because in 2010, my dx was amended to Secondary Progressive (SPMS), and prior to this there were very few new treatments, but now, the majority of new treatments are for those who are newly dx'd with RRMS. I've not seen anything for SPMS.
About 6 years ago, my son (then aged 21) told me after he'd had a nasty fall one morning, that he'd been experiencing some of the same problems that I have with my MS, and he was worried that he might have it too.
Now, it is known that children of MS patients are more likely than the general public to get MS, but even so, that is still a tiny, tiny %age of the population, but just to be sure, I took him to see my GP (who had looked after me for years), and explained his dilemma.
She did some basic tests, which were obviously inconclusive, but didn't rule MS out, so she referred him to my neurologist, explaining his symptoms and pointing out that he is my son, and he was quite quickly called into the hospital for a consultation, then an MRI, etc.
Thankfully, his tests came back negative, but we were very nervous for several weeks, hoping it was all a false alarm.
So basically, what I'm saying, is that even if your sister does have MS, the prognosis is generally pretty good for those newly dx'd as there are far more treatment options than there were 29years ago when I got my dx.