Virtually anyone can get an EFIN https://www.irs.gov/e-file-providers/faqs-about-electronic-filing-identification-numbers-efin 

You are an accountant working for a family... We prepare all of the .... = SOUNDS LIKE A PAID PREPARER!!!! You may or may not also need a PTIN https://www.irs.gov/tax-professionals/ptin-requirements-for-tax-return-preparers

Consider ProConnect Tax Online as a closely held software that the family or you (who sounds like a paid preparer) can use to to the PF.