In England, the La assume any money in joint accounts is half and half. Therefore, if the account had £100k in £50k would be 'allocated' to your sister and used for her fees.
At the next review, usually a year later, the same thing will happen. So, if £20k has been been spent, the remaining £80k will be split in two again meaning that the husband has actually paid for half your sister's fees.
Arguments can be made about who contributed what proportion over the years.
AgeUk have good downloads available on line. CAB can advise too.