If you live in Boston, you need to file for guardianship of a minor at the Suffolk Probate Court. I've posted a link to the Probate Court's web site, which has most of the forms you need, as well as a link to the Court's on-line self-help center. Keep in mind that you will need to get the written assent of the minor as well as the parents.
You may not need temporary guardianship if the mother has the sole custodial authority. If it is for educational and medical reasons only, then there is a potentially quicker, less expensive way to get authority. Massachusetts has recently enacted the Uniform Probate Code. The new law allows for the execution of a caregiver authorization and affidavit. A parent can appoint a caregiver who resides with the child (by written, signed and notarized instrument) with authority to assume parental responsibilities relative to a minor child’s education and medical care for up to two years. The new forms should be available on the Massachusetts Trial Court website - http://www.mass.gov/courts/courtsandjudges/courts/probateandfamilycourt/forms.html
My answer does not create an attorney-client relationship.
I agree with Attorney Rockwell that you may not need to file a guardianship petition when there is a caregiver authorization which is properly drafted. You can also apply for a limited guardianship so that your niece's mother retains authority to make decisions in certain areas.