It's not selfish to take care of yourself and your own needs. It's self-care and is very important. It's taking me a long time to see what my needs are. Listening to your instincts and your body are great tools for self-care.
Congrats on the Alanon meetings you have been to. If you have a phone list, I'd suggest reaching out for help, or call the numbers listed for contacts for your local meetings. If you keep looking, something may come about to help with transportation or physical help. Your local Domestic Violence center may also be a great resource.
It doesn't matter whether your sister is alcoholic or not, or temporarily sober or in recovery, giving her a place to stay is taking over what is hers to deal with. It doesn't sound like a good situation, even for one night. Are you scared of her reaction if you say no?
There are many programs, including the Salvation Army. There is no shame in needing help. Within the confines of dysfunctional relationships, my ideas of "helping" have often been very unhealthy. Programs like these help people to achieve more, if they're willing to do the work. If they aren't willing, nothing I could have done or said would have saved them.
The Salvation Army - Adult Rehabilitation 