I am not licensed in North Carolina or Maine, but there is a lease agreement which obligates the tenant to pay rent (and the landlord has obligations as well). That agreement does not stop or is not cancelled because of a tax lien.
The lien just reflects the taxing authorities interest in the property. For example, if your sister sold the trailer, the lien holder gets paid out of the proceeds. It does not affect the obligation to pay rent.
If the taxing authority is to levy and seize the property in order to sell it, then that could possibly be part of the issue where the tenant is forced to vacate the property; but that does not sound like what is going on.
Your sister should probably contact a local attorney in the area near where the trailer is located to look over the situation and follow up with the tenants. There may be some legal clinics in the area that can also help for free or low cost (check with the bar association).
This answer or response should not be considered legal advice, and does not create an attorney-client relationship. If you have further questions, I would be glad to discuss your situation further. I can be reached at US - (801) 746-6300, or online at -- http://www.lewishansen.com/attorneys/robinson.html