The simple answer is you can reach out to DSHS/cps. That advice is simple
but the consequences of doing such is not so simple.
Your cousins could be encouraged to reach out to CPS themselves or talk to a trusted teacher or guidance counselor. In most places police officers are mandated reporters of child abuse or neglect. If you and your mother really want CPS involved with your cousins, a call to a ranking officer at the police department should prompt the responding officer to at least report what your mother told him to CPS so that they might begin an investigation.
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Call CPS as a starter. That is a hit and miss operation.
You might speak to a family law/probate attorney. Any other posible options depend on the ages of the kids.
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