Hi there Cactus (such a nice username, how I envy you) and very welcome to CS. I hope you and your sister will have lots of fun playing CS together^^. Don't worry about getting banned for sharing a computer, CS doesn't care how many users use the same computer as long as the trades between them are somewhat fair. You don't need to have the note in your signature, the mods will ignore it (or possibly remove it) anyway^^.
If you havent already, you might want to check out these topics for new players:
Also, have a look in the
CS Help section and the
Guide Index, you can find all sorts of useful links there!
If you have any questions, me and everyone with a pink name are
general helpers (full list of names
here) and we are here to answer all your questions^^. Don't hesitate to contact any of us if there is something you want to know^^.