Your sister is likely to arrive at a B or C gate and her flight to BOS will almost certainly depart from a Z gate. So you want to be able to get into Z departure area to meet your sister.

Things may have changed but there used to be boarding pass checks before security if you are departing from Z. In order to be certain buy a refundable ticket for a flight departing from Z. Alternatively you could get a ticket for a Schengen destination and enter A departure first and go through passport control into Z. Passport control doesn't usually check a boarding pass. Also if you need a visa to enter certain countries make sure to choose a destination not requiring a visa.

You can check Lufthansa's flight status page or Frankfurt Airport's departure information page for flights departing from Z gates. Good luck!