If your sister speaks to her creditors, and tells them that she can't meet the repayments, she'll be able to get the interest on most (or probably all) of the accounts frozen. If she doesn't, the amount she owes will keep going up, making her problems last even longer.
Credit card firms, loan companies and banks never write off debts unless, for example, a debtor dies leaving no money and nothing of value. If your sister doesn't voluntarily enter into repayment agreements with her creditors, they'll seek county court judgements against her. The court orders will force her to repay her creditors. (If she attends the court and completes all of the relevant forms she won't be ordered to pay more than a realistic amount each month. If she ignores the court proceedings the court might order that she repays the whole lot at once).
If your sister ignores the CCJs, the creditors can then seek to enforce the judgements by, for example, getting a court order to have money taken directly from her earnings.
The CAB can be helpful but they've often got long waiting lists before someone can get an appointment with a specialist debt advisor. CCCS is a charity which specialises in helping those in debt. Their services can be much quicker:
http://www.cccs.co.uk/
Chris