Jambo Palle - your sister will enter Tanzania on a regular "tourist visa". She will then need to apply for a "non-resident C permit" which permits her to be involved in volunteer activities. She should contact the organisation she is volunteering with to confirm the C permit arrangements. The visa fee for the second permit varies between $100 US and $500 US depending upon where you are working and what you are doing and who you are volunteering with. Generally a three month C permit should cost $200 US (on top of the $50 she will pay for the regular tourist visa). What happens if you do not apply for the second permit (I hear you ask) - well nothing, if you are not caught. But if you are caught - it can lead to a night or two of confinement (jail!) and deportation, with her passport stamp "not valid for entry into Tanzania" for up to 5 years. The application form for the visa and current fee structure is available on the TZ immigration web site (or at least it use to be).
I cannot say specifically how strict the immigration officials are "currently" as I have not been there since 2012. But they do call at all the regular backpacker accommodation places and anyone who has been a resident for more than two weeks usually gets checked up. Hope she has a great time - its a country that usually appreciates volunteers who are there to make a real change or improvement - not to mark of something else on their bucket list.