Hello again, Justin, and thank you for this additional information.
Your brother-in-law should typically file an ex parte application for immediate custody of the children. He must file an affidavit with this application setting out facts describing why the children are in immediate danger in order to justify the court's entering such an order without notice to the other parent normally.
Here is a link which provides information on how to file such a petition, along with the forms that need to be filed:
Once he obtains an order of custody, then the police can aid him in enforcing it and issue a bulletin to other departments in an attempt to locate your sister.
It is strongly recommended that your brother-in-law retain a local family law
attorney to obtain this emergency order for him as such orders are disfavored by the courts and not easily granted since it impinges on a parents' constitutional rights.
The state and local bar associations can provide him with attorney referrals if he needs one.
I hope this helps clarify the situation for you. Please remember to rate my service once you have all the information you need so I will be compensated for my time from the deposit you posted with this website. If you have any other questions, please ask me – I’ll be happy to respond. Thank you!