I suggest getting the insurance through an experienced broker, as cover for houses in probate is a bit of a specialist area.
The executor has to apply for probate, which involves collecting all of the relevant information together, completing the paperwork and swearing an oath (or simply 'affirming' if, like me,he's an atheist).
The 'grant of representation' that he'll then have entitles him to distribute his mother's estate in accordance with the terms of her will (and in accordance with the intestacy rules if, as explained above, a partial intestacy has been created through his sister's death).
At that stage though the house will still belong to his mother's estate and not to him and his brother. However he'll then be empowered to 'assent' the property to the beneficiaries (i.e. himself and his brother), using Land Registry form AS1:
http://blog.landregistry.gov.uk/property-owner-dies/
Once form AS1 has been accepted by the Land Registry, and the title register changed, he and his brother will then co-own the house.