My sister has a projectile vomiting condition, and rather than going to the bathroom when she first feels sick, she waits until it's too late. I'm getting frustrated with cleaning up after her. How can I get her to go to the bathroom at the first sign?

Does your sister have other conditions, such as dementia, that make it hard for her to know what to do? Does her body give her warning that this vomiting will occur? If she has no reason why she can't handle this better, it does seem like a horrible thing for you to deal with.
Even if she has a legitimate reason for not knowing it's going to happen, cleaning up after her all the time has to be horrible. Is everything being done medically? I'd seek a second opinion from a doctor. You may not be able to keep on caring for her in a family setting if this can't be handled better.
Good luck,
Carol

Get a small waste basket and put in a plastic liner, maybe a plastic grocery bag. She keeps the basket with her, barfs in it, closes the bag up and throws it away. New liner, new barf bag etc.

We have that already. Thank you for the idea though. The problem is, (and this is according to the psychologist, not me; however, I agree.) She doesn't care anymore. She's been a slob her whole life. Now that she has to be here, she thinks she can still do what she did at her home. The waste basket is there and if she doesn't feel like picking it up, she won't. She will wait until she starts spewing into her hands. Even if I am standing there holding the the basket in her face. We anticipate moving soon and then she will have her own room. She says, "I can't wait to have my own room." Neither can we, but she is not going to let it be a pigpen of a room, there either. She has many "mental health" issues. The psychologist at the last visit, says she knows how to play the game and manipulate me. I was told to start calmly telling her that what she does in inappropriate and unacceptable. OK, I get that and I do. But this is years of unkempt issues that I am tryin to wipe from our home. We are not going to allow her to trash our home as she did hers. It cost her $3400 to have a professional cleaning service come in and clean it, fumegate it and make it presentable to the next people that would live in it.