BIL stole Mom's disabled parking placard. Sis has POA and won't do anything to get it back from husband. Any advice?

They won't cooperate. Do we report this to the DMV (we're in California) or call the cops? Mom is being held prisoner in her own home. We got the disabled placard so we could take her to her appointments, shopping, etc, and have always kept it in her home in a place where anyone that was driving mom could find it. BIL says it's his now because he started taking mom to appointments a few months ago. The jerk deserves more than just a fine and mom wants to get out of the house. When 2 of us are with mom, it's not a problem - one person gets out with her and waits while the other parks and catches up. No big deal. But it's rare mom has 2 people visiting at the same time. If we had POA, we'd just cancel it and get another.