You can report the tenant to the housing authority that administers their section 8 voucher. The housing authority may drop by to investigate the allegation without notice to the tenant. The tenant will likely claim that the sister and the kids are just visiting (which is allowed), so you have to be able to prove that the sister and kids are staying overnight for more than just a day or two at a time. You might also want to send a notice to quit to the tenant for breaching her lease and start a formal eviction process; the fact that you see the kids and sister moving there is evidence that can be used for the eviction. Seeking a private party complaint for trespass in the local district court is risky, as it open you up to a counterclaim by the tenant.