Mass. Gen. Laws ch. 217 § 6B
No justice, circuit justice or special justice sitting in a probate and family court shall be interested in, or be benefitted by, the fees or emoluments which may arise in any matter pending before any probate court or which may arise in any suit or action pending in any court of this commonwealth where the subject matter or cause of action is founded upon or derived from proceedings begun in any of the probate courts nor shall he, except as otherwise provided, be appointed or act as executor, administrator, guardian, conservator, trustee under a will, commissioner, appraiser or assignee of or upon an estate within the jurisdiction of any probate court; nor shall he be interested in the fees or emoluments arising from any of said trusts. Full-time justices, or circuit justices provided for under this chapter, shall devote their entire time during ordinary business hours to their duties and shall not, directly or indirectly, engage in the practice of law. The judge of probate for Dukes county and the judge of probate for Nantucket county shall sit in the probate and family court for any other county upon designation thereto by the chief justice for said department pursuant to section eight, and when so assigned shall receive from the commonwealth the expenses incurred by them.
Mass. Gen. Laws ch. 217, § 6B