If you don't want to lie then how about just not answering the door when she knocks? If mom is inactive then maybe you can just tell mom that you really don't feel like studying with sister so-and-so because all she does is talk and you really aren't getting all that much out of her "time". Take mom shopping or to a movie that day. Hide under the bed. Get in the shower when she approaches the house. Answer the door in your underwear. Or if you just can't do any of the previous suggestions with a clear conscience then invite her in and (are you over 21?) open a beer and sit down to study (that is if you drink beer). Okay, so maybe that is extreme, but don't lie. Either be straight with your mom about how you feel or be straight with this sister. After all (and everything that I have read on this site) do you think she is all that jazzed about studying with you? (not that you are not a nice person and all, but a lot of what I read here is maybe this is just "service time" for her as well, then again maybe not). Don't sweat it. If all else fails and you cannot do any of the above or of the others suggestions...how about not coming home till Saturday?
Above all else try not to lie. Avoiding is okay, but don't lie.
For what it's worth. (not much probably)