why couldn't you do it? If they said they'd pay and then backed out that's not really fair to you. You're willing to donate plasma but you aren't willing to say no?
If they don't pay they don't get to stay; it's not your responsibility to find them a way to pay you, it's theirs. Sorry if that's harsh, but regardless of friendship or family people will take advantage of you if you let them. Either you're ok with them not paying or you're not.
I'd suggest helping your sister get a social security card, she's going to need one and there's no reason she shouldn't try to get one: http://www.ssa.gov/ssnumber/
and I'm sure your friend has at least a -little- spending money, a few dollars here and there. Tell her to save what she can out of every paycheck. The con is months away. If she can't do that then she shouldn't have agreed to help pay for the room; I'm assuming her situation with her mother isn't out of the blue, so it shows she never intended to help. If you can afford it by yourself and she can contribute in another way, say buying food for the weekend or something, then you could compromise but letting you book the room without ever intending to pay her part is not cool.