When a person who is out on bail for felonies commits another crimes, often times the bail on the first charge is revoked. It is not a gurantee, but it is often the practice. In your case, there the brother-in-law was already arrested for felonies and youi have provided such clear and convincing evidence of subsequent felonies by him, it does seem unusual that the police have not picked him up yet on the new charges. Either they do not have enough evidence yet or he is cooperating with them in some way and they are giving him a break. It certainly sounds like they have enough evidence to arrest him for the bobcat theft. As another attorney indicated, you have the right to seek a civilian compliant against him. The other attorney did a good job in explainting the process so I won't repeat it. It is certainly your choice as to how to proceed. I would be skepitcal of the police's actions as you are given the evidence that you have provided already. There certainly sounds like enough for a Clerk of Courts to issue a criminal complaint on the civil application. What you have provided sounds like enough evidence for probable cause to me.
When someone is arraigned on a criminal charge they are usually given what is called the bail warnings. That means that should one commit a new charge while awaiting trial their bail could be revoked and they could be held for up to sixty days fined or both. In order for someone to have their bail revoked the prosecutor must prove that 1) they got the bail warnings in a prior court event 2) there is probable cause to show that they committed a new offense and 3) they are a danger to the community. Whether or not theft of something constitutes a danger would be a question for the judge. The judge may or may not hold the person in a jail and may release him.
It is the police officers decision to arrest the person. If you are unhappy with the speed of the investigation you can always go to the court and apply for a private criminal complaint. If, after a hearing, the clerk issues the complaint your case would be prosecuted by the district attorney. In that case your brother in law would be summons to court rather than arrested.
Hopefully that helps.
Commission of a new offense with an open case is grounds for bail revocation, but bail revocation is not required. The prosecutor must first motion for bail to be revoked, and the upon the prosecutor's motion, the judge must decide that the statutory criteria has been met and elect to exercise his/her discretion to revoke bail. The motion for bail revocation is heard on the defendant's first appearance before the court on the new charges, i.e. the arraignment. If your brother-in-law has not yet been arraigned, there would not have been a motion for bail revocation heard.
While it is not a very common practice when there is a felony involved, the police may seek to go forward by way of clerks' hearing and a summons arraignment, rather than arresting the suspect. Keep calling the detective on the case; the squeaky wheel gets the grease.