This is a thorny problem; A spouse has a right to claim part of an estate. However, usually if they don't claim their legal rights within specified time limits, that is the end of the matter. In this case, because the spouse lacks mental capacity, it is possibly the case that the time limit would not be deemed to be running against him. However, this should not stop a sale: it just means that one third of the nett sale proceeds should not be taken by your husband and his brother, but should be placed on deposit by the executor for the benefit of the husband. (actually it should be paid over to the husband perhaps, but at a minimum it should be placed on deposit).
If your husband and\or his brother are actually the executors, they have more room for manoeuvre. If the solicitor is executor, there is less room. But either way, there is no technical obstacle to going ahead with a sale. (oops, sorry- one small obstacle: if the house is less than one third of value of the total estate, then the huusband can claim the house itself in satisfaction of his share, in which case you really are bunched)