Can my sister get out of the house on her own or with some girl friends alone without a guy?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
There is no objection to your sister leaving the house when accompanied by trustworthy and reliable women. Knowing her destination is highly recommended. In addition her going out should be limited to necessities, such as visiting relatives, attending permissible gatherings or seeking knowledge, etc. If there is no necessity, then she should remain in her house as Allah commands them "And stay in your houses…"[33:33] Know that a woman travelling without a Mahram is not permissible. For more details, please refer to Fatwa No.: 81210 and 81435.
Allah knows best.