Does the Residuary estate go to all siblings on the will or can it go on a verbal agreement with the mother and all siblings present that divides the property equally?

The residuary estate is usually addressed in the residuary clause in the will. If the residuary estate is not addressed in the will then the residuary passes to the next-of-kin as intestate property according to the laws of intestacy. Distribution of an estate is governed by statute and not by any verbal agreements. A beneficiary can waive their rights to distribution but it must be done in writing and filed with the estate.