After the executor has been confirmed as such by the probate court, consent of the beneficiaries is important because it will help persuade the judge to give the executor permission to do whatever it is the executor and the beneficiaries are asking for. This certainly is so as a general matter, if there are circumstances (which in frequently occur) for which an executor ought seek and obtain court approval for some transaction affecting estate assets; and (as noted) the cautious/punctillious executor will often want to obtain such judicial approval. Correlatively, however, there are some U.S. jurisdictions in which this sort of thing is neither common nor required (and some circumstances in which a court might, for policy reasons, decline to provide approval which, however, might not be necessary anyway).
In some cases, moreover, not merely is seeking such judicial approval thus not merely superfluous but can also be an occasion for abuse, e.g., if what actually are avoidable attorneys fees are nevertheless incurred (yet there have been cases in which some estates lawyers recommend such an application mostly to generate such fees for themselves), especially when (as the OP seems to suggest is so for him and his sister) the beneficiaries, one of whom is the executor (whether already approved/appointed or only still nominated), have good reason to trust one another and wish to act in what they all agree is a practical manner.
But even with consent of the beneficiaries, the executor cannot sell land or in any other way dispose of estate assets without the court's consent.
The "cannot" often applies if the person nominated as executor has not been appointed or otherwise qualified as such, since s/he would not have the authority in such circumstances to convey title if the property sought to be conveyed is then still titled in the name only of the deceased. But if the executor has been formally appointed or has otherwise qualified -- e.g., if "Letters" or like jurisdiction-specific authority have been issued -- why is "the court's consent" needed (or, in some places, even likely to be available) if the transaction is "with consent of the [if: competent] beneficiaries"?