How do you evict someone without a rental agreement?

There are two different options:

1) If the agreement with your sister to live there and pay rent was firm *and* she made at least one of the payments, best would be to treat the other people as subtenants or guests of your sister, since a rent-paying tenant, your sister, moved them in. You therefore evict your sister for some good cause (e.g. nonpayment if she--presumably--stopped paying rent at some point) and when she is evicted, her guests and subtenants have to go, too, since by getting the "right" to stay there from her, their right to occupancy ends when hers does.

2) If there was no rent payments at all from your sister, and especially if she herself never moved in but only moved in her friends, she would likely not be considered a tenant: under these circumstances, it would probably be considered that there was offer to let her rent, which offer she rejected. In that case, you bring an action for ejectment (basically eviction for nontenants) against her guests. Ejectment is how you remove non-trespassers who are not tenants: since your sister would presumably, even if she did not pay rent, have at least some or arguable right to possess or reside in this house, and since your parents did not instantly call the police on these peopel as trespassers, after 2 month, they would no lot be considered trespassers.

However, the exact best way to proceed is something your parents need to discuss with a landlord-tenant attorney, because this is a  more-complex-than-usual factual situation; they need a lawyer to evaluate things in detail and they should let a lawyer represent them: not only will the lawyer present the case propertly, but he will provide a buffer between them and the occupants.