Teenage girls are going to want lots of data for pictures and video. I would highly recommend getting the phones unlocked and buying SIM cards on arrival, and even if your sister sticks with the international plan instead of getting a SIM herself, they can all use Whatsapp in lieu of texting, which will save a lot of money.

You can read up on the different options here:

http://prepaid-data-sim-card.wikia.com/wiki/Germany