Whilst I am aware that under the "Fuel Direct" scheme, the utility payments can be deducted from some benefits (but not DLA) it seems that water is NOT one of these utilities. This is because under the Water Industry Act 1999, water companies cannot disconnect a domestic customer for non-payment of water charges. Water charges are therefore no longer a priority debt.
Please see what ofwat says about water rates arrears:
Payment options
Each company in England and Wales
must have a code of practice on dealing with customers in arrears, and all companies must provide information about the code on their bills. The code of practice should be used when negotiating with the company. And the
negotiating is the main tool here. If your sister is in receipt of disability benefits, she may be entitled to one the "vulnerable group" schemes- see the ofwat link, the right hand side menu "Having problems paying your bill"
Many water companies in England and Wales have internal hardship schemes or fund independent charitable trusts which can help with water and/or sewerage debts.
If your sister's water company is awkward- contact
Consumer Council for Water : Consumer Council for Water regional committee as CCW is independent of the water industry.