It depends very much on your personal situation. Some cats are loners and will not take kindly to an interloper be it one or two. In this situation if you are determined to get another cat, I would get two so that the new comers have each other since the original cat is unlikely to befriend either of them. If you only get one in these circumstances, the territorial first cat will shun the other and it will likely never feel at home, but if it comes in with another can that it already knows from shelter life, it is likely to manage OK. First kitty will have to have lots of extra attention so that he/she doesn't feel replaced.
If the first cat is social like my cat Hank, I think he would do better with jusone cat at time. Does the shelter allow for you to adopt one and wait two weeks before you pick up the other? I can't say for sure, but I would think introducing them one at a time to a social cat so that it doesn't have to feel like its choosing one buddy over another lide I think my cat would feel. When he has friends over in the yard, it is only ever one cat at a time although there are two different ones that visit regularly.
Every cat is different and every pet home is different. It sounds like the cats will have two different owners, you and your sister. In that case I think it is OK to have the two cats right away because there is more than one person to give attention to all the cats. None will feel left out. Good luck. I would like to hear how it turns out.
