At minimum, you and your siblings need to be on side. You need to sit down and make a formal plan with them about what they can and will do, and then none of you can budge. This doesn't mean you have to cut her off; it means that you have a plan. Perhaps the plan is that your sister pays rent and you each give her £50; perhaps it's that you will only help her in-kind with food. But what it means is that boundaries are set. It makes saying no a lot easier.
At best, you do that and then sit down with her and have an intervention of sorts. She'll hate it. Too bad.
You can also make support for her conditional on her receiving debt help. Sometime like, Amy and Ben and I have talked, and we think you need serious help. If you go to CAB/whatever, we will each help by paying X/month to your debt/bills. But as it stands, this situation is only going to get worse, and we can't do more.
Again, she won't like it. Too bad. She'll probably get angry, defensive, and lash out. You have to ignore it and stick to the party line.
You'll feel a lot better when you feel like you're helping without being taken advantage of. You'll also feel better when you can say a clear no.
But first step: actually make a plan with your siblings.