Father died last year, no will, 6 daughters and oldest has taken over the estate but youngest daughter a few months ago, had his truck hidden and just sold it and spent the money from sale, she forged his name on title of truck. What can I legally do ? (I do not know the VIN or plate number)
I assume you mean that your father died, then one of your siblings just acted unilaterally. The specific details are governed by the laws of your state, but every state has provisions for "intestate succession" i.e. when someone does not have a will. In every state, somebody would have to petition the court to make them be "in charge", and then they have to follow the law in disposing of the estate. First you have to settle up any debts, then you can divide the remainder between the legal beneficiaries. Let's say that that means "the 6 sisters".
The person in charge gets to implement the law, so could give the youngest sister the truck as (part of) her share. Let's now say that the administrator-sister did not do this: she has a duty to protect the interests of all the beneficiaries. That would mean "subtract that amount from what sister 6 gets", and does not necessarily mean "and file a criminal complaint against sister 6". When all is said and done, if you don't get your fair share, you would have a civil case against sisters 1 and 6, so you could sue to get the money back. There are probably procedures in your state whereby all beneficiaries have to sign off on the disposition of the estate, though the court probably does not affirmatively track whether the paperwork has been filed and go after the person in charge. So you may have to hire a lawyer to get anywhere.
The police / DMV might be interested in document forgery.