Now that probate has been issued, the executor has a legal obligation to discharge the will in accordance with its provisions.
The house may form only part of the estate and there may be bank accounts etc that the executor will have to access and close down.
As part of his/her duties, the executor has an obligation to dispose of the house, effectively generating an amount that is added to the value of the overall estate. This then has to distributed in accordance with the will’s provisions.
As
@Clamball has said, the easiest option is to simply register it in your four names but this doesn’t really help much in the long run.
There’s no set period within which a will needs to be administered after probate but anything beyond a year would be unusual. Where an executor is dragging their heels, it’s usually the beneficiaries who insist on progress being made.
The bottom line is that it’s not really up to your sister to get her act together. The executor has responsibilities to all beneficiaries and can insist that she make the house available to sell.
Other than for LPT reasons, I don’t think Revenue care in whose name the house is as long as long as all due taxes have been paid.