No, it has no bearing on the long-term outcome whether you receive prompt treatment, or indeed any treatment for an MS episode, AKA relapse. It’s not a medical emergency - with a few obvious exceptions, such as if the person experienced trouble breathing, taking food or water, or was unable to pass water.
Steroids are the only treatment usually offered, and these may help speed up recovery, but they come with quite a few risks and side-effects, and there is no evidence they improve long-term outcomes, so there may be an argument for giving them a miss - particularly if the episode is mild. Not all neurologists are enthusiastic prescribers, for those reasons.
Really, the only compelling reason for seeking prompt intervention is just in case the patient was mistaken, and whatever it was wasn’t their MS playing up! Another poster here once wrote - I don’t know how true it is - that the commonest cause of death among people with MS is ignoring something that wasn’t their MS. People who are used to living with chronic illness tend to assume everything’s down to that. But of course, we aren’t immune from other (serious) medical issues, so we can be at risk of ignoring something serious, on the basis it’s “only” the old MS again.
Particularly if symptoms are different and unexpected, compared to anything that’s gone before, it’s wise to check they are, in fact, due to MS, and not something unrelated.
The only other reason to get a relapse on record, even if you aren’t offered or don’t seek treatment for it, is that frequency and severity of relapses can have a bearing on which disease modifying drugs a patient qualifies for. Not all patients are on disease modifying drugs at all - some don’t want to be, and some don’t qualify. But if relapses aren’t recorded, they can’t count towards eligibility for a DMD - or a stronger one, if already on one.
So it’s not so much about relapses needing “urgent” treatment, as (a) making sure it really is a relapse, and not something else nasty that has nothing to do with MS, and (b) making sure the hospital is in the picture that a relapse has occurred, so this can be considered when planning future treatment.
Your sister won’t have come to any harm from not receiving steroids immediately - or even if she never had them at all.
I’ve never had steroids for any relapse - but that’s largely because most of them occurred before I was diagnosed. I was never offered steroids for the more severe relapse that eventually led to my diagnosis, and I never asked, because I didn’t particularly like the sound of them anyway. I’ve always said that my “red line” is if I ever couldn’t see or couldn’t walk. Then I’ll take the steroids, as it’s important to get better as soon as possible. But short of that, I’m prepared to let time and patience do the work. There’s no evidence you finish up in any worse position than if you’d had the steroids.