You're unfortunately leaving out part of the equation. Did your mother die with a will in place or intestate? It sounds like you might be making assumptions about what rights you and your siblings have to the property. How and to whom the property passes will greatly affect the answer. Please provide some additional detail.
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If she died without a will (or "Intestate") and your mother was not married at the time of her death, you four children can execute a deed basically saying: "you, brother, sister and sister, as the sole surviving heirs of the Estate of Mom" convey the property to "brother" - and you can do this without the considerable time and expense of Administering your mother's estate or seeking judicial approval.
That said, you and your siblings seem to be acting very generously. You are giving up what may be a valuable asset, for no consideration. If you are all convinced that you wish to do this, you should speak to a Real Estate attorney. This can be done relatively easily and inexpensively.
You should have your mother's estate probated (if she left a will) or administered (if she didn't leave a will). Then the sisters could give the property to the brother if that is their desire.
Although it is possible to do this without getting the courts involved by just having the sisters execute a deed in favor of the brother (as the sole heirs or devisees of the mother) the title that your brother would get would always be questioned. It is better to have the court give it's approval, thereby ensuring that your brother's title would be clear and insurable. It is important when he wants to sell or refinance the property.
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