That is the hurdle you face...law enforcement (including the police and the county prosecutor) have "discretion"
They have the ability to decide what they will and what they will not spend resources investigating/prosecuting.
There is no legal way to force the police or the county prosecutor to investigate the case.
That said, you may still have some options.
I agree with you...if you had personal property stolen that is a crime...they should investigate. I would go "up the chain of command" to the chief of police and ask them to do just that.
Then, if the chief refuses, you can contact your elected representative (council person for the city) and ask them to put pressure on the police. The police are part of the executive branch of the government....they work for city council and the Mayor.
If that does not work?
Then you can use civil court...you can file a claim in small claims
court against your sisters for the value of the property they stole from you.
BotXXXXX XXXXXne: you can not force the police to prosecute. But you can ask them to...and you can also file a civil lawsuit to recover the value of the property they stole from you. If you file in small claims you can save the cost of a lawyer and "do it yourself"