Hi Chloe2u,
No, the executor cannot remortgage the house, even if she has title to the property,
unless the Will gives her power to. However, the Will should have a provision where any actions taken that affects estate property, including the house, must be made in the best interests of the beneficiaries. If the house is left to 8 beneficiaries, the executor must consider the interests of all 8 beneficiaries, not just one. Certainly, remortgaging so that one brother (presumably, a beneficiary) can build a structure on the property, is NOT allowed. One of the (many) reasons is because the beneficiaries may choose to sell the property and split the profits, provided the Will allows for this.
Again, it is very difficult to advise without knowing what is written in the Will. This is because the executor's powers come from the Will and not from the grant of probate afterwards. However, the executor should not be dealing with estate property before a grant of probate, unless there are immediate pressing matters or the Will provides for an immediate legacy/gift. Even if the Will provides for an immediate legacy/gift, the executor should still wait for probate because there is a possibility that the Will is not valid, or the provision she is effecting, is not valid.
Anything the executor does, before a grant of probate, may render the executor personally liable
if it is later found out that this was not in accordance with the Will, an improper exercise of executive power, the Will or its provisions are invalid. And as
@Rod says, her legal costs to defend against such actions will not come out of the estate but be personally born by her.
In short, your sister would most likely be misusing the title deeds.