I've known about Pokérus for a while now, but I only got my first infected Pokémon in Y. I don't totally understand EVs, but basically they affect which of your Pokémon's stats get raised when it levels up. You can only have something like 252 EVs total, and they're spread between each of your stats. Stats with more EVs go up more. All the Pokérus does is make EV training quicker, but thanks to super training, you don't need to EV train, 'cause it does the same thing only easier. So... as long as you supertrain semi-properly, you don't need to worry about your sister beating you. Unless you're just generally rubbish at Pokémon. :p