The incubation period is short. Usually 1-2 days. If your sis has the rash now, and it's been a couple days, you're not likely to get it.
Prevention of scarlet fever is the early treatment of strep throat. Bacteria are spread by direct contact with infected persons or by droplets exhaled by an infected person. Just like a cold. Avoid contact with infected persons.
So if you get a sore throat, you'll know to get treated right away.
You can get it when you're older even though it's less likely. I had it when I was 29. It's really not a big deal anymore. I mean that you're no more sick with scarlet fever as you would be with strep throat as that is what causes it. You'd just look a bit goofy with the rash.