As others have indicated, the value of the estate is a great deal more than £500, since the house forms part of the estate.
It's essential that the executors apply for probate otherwise the ownership of the your mother's share of the house will effectively remain 'in limbo'. That will cause considerable problems when your sister dies.
Obtaining probate is an easy process. You do NOT need to use a solicitor. Unless there are unusual complicating factors (which there don't seem to be in your case) anyone with half a brain can do the form-filling on their own. Probate Registry staff are incredibly helpful. The only payment is a £90 application fee but you might need a few additional official copies of the grant of representation. Since they're only £1 each (if ordered at the same time as the application is made) it will cost less than £100 to avoid some very complex problems in the future.
Speak to the staff at your local Probate Registry. They'll guide you through the process. You can find your local registry here:
http://www.hmcourts-s...robate/registries.htm
Chris