This is so, so hard. You and your parents meant well but, unfortunately, what your sister learned from having so much generous help is that she doesn’t need to help herself. She has figured out that if it looks like she’ll “end up on the streets”, you can’t stand it and will help her even though you said you wouldn’t. She isn’t going to believe you now unless you can figure out how to make it clear to her that the gravy train has stopped.
Since you are older than she is, you are close to retirement. You probably can’t support her anymore and be able to manage a reasonable retirement for yourself. It’s long past time for you to be more selfish and to take better care of yourself. That means steeling yourself to let your sister deal with the natural consequences of her own behavior.
Someone who “can’t be bothered” looking for a job and who won’t apply for benefits is creating her own crisis. She may have to be homeless before she gets it that she has to do what other adults have to do. If she does become homeless, you can give her the phone number of the public services that help people figure out how to apply for benefits and get housing. You can’t cave in — unless you are willing to continue to take care of her at your own expense until one of you dies.
You may need some support to hold onto while you make such a major change. For that reason, I do suggest you make an appointment with a therapist for some help.
I wish you well.