If you both have the same e-mail address then what you want to do is create a new e-mail address for yourself. Then go on Neopets, go to the neoboards and at the top click on User Preferences. Then go to Change Your Email and you enter your new e-mail adress along with your password and it will be changed. You'll then be on a different e-mail from your sister (you could change either yours or your sisters - whichever is easier for you).
On your lookup if you want to say that there are two people playing neopets in your house just follow Xerophyte's instructions - go to User Preferences again, and this time scroll down to the big box beside About Me. There just write in what ever you want to put on your lookup - for just plain writing that is center aligned you could try entering this:
Code:
<div align="center"></i><br>To TNT - please be aware that both me and my sister play Neopets from this IP, thank you :)<br></div>
Just write whatever you want or play around with the above. The div part is just telling it to be centre aligned which looks nice, the <br> are like hitting return on a keyboard and the < / i > is telling it not to be in itallics as it will try to otherwise. Sorry if you already knew all that