My mom is on parole and lives with my sister. My sister bought a car and my mom was supposed to cosign. My mom ended up as owner and my sister cosigner. Every time the plates get run my moms parole officer calls my mom. She knows my mom is home by her curfew time,she's been complying in everything. The po is telling my mom that if anything illegal happens with the car,(which my sis is cosigner on and she drives it past my moms curfew) that my mom can get violated. Is this true?

Parole officers have a wide leeway. Your mom needs to carefully review the terms of her probation. If the car is out after curfew its possible to be violated.