Just like appraising..tax or any consulting can be done by anyone really. It is who will accept the persons work...whether it be determining value of something, or what the tax effect of something might be. If your consultant is wrong..you bear the consequences, not them. (You may accept the appraisers work...but will the bank? You may act on the tax advice, but the IRS doesn't have to agree).
In some places, someone may need a liscense to hold hemselves out to the general public, or for it to be acceptable for certification, to perform certain services. Which is to say, your brother in law can do your electrical work if you want...but for the town to certify it for use, they may require it be done/reviewed by a Lisc. electrican. And he may not be able to advertise he's in the electrican business.