Police officers code of conduct/can the police begin an investigation?

Lets just say that there's this woman, who has a 20 year old son who lives at home. After searching the sons vehicle while he was away, she discovers that he is in possession of at least four grams of marijuana.After confronting her son with the evidence, they argue, and he takes the weed, and disposes of it.This woman has a stepson who is a patrol officer for the metro police.All though the son has not told his parents who he purchased the weed from, the mother has a suspicion that it is an older cousin of the boy, because she knows that this cousin admittedly smokes marijuana. There is no other evidence that the two cousins have anything more than a friendly social relationship, but this woman and her husband are attempting to gather info on the cousin (home address, lic plate, workplace) in order to give this info to the other son who is an officer, and most likely to attempt to cause trouble for the cousin who they firmly believe is the reason that their son had weed in his possession. The woman has already been in a family dispute with this cousin when a relative ped away a few months prior, and has already made many pive-aggresive actions concerning this cousin.(non invites to family functions, withholding of sentimental property of dead loved one, rumor spreading among family that the marijuana smoking cousin is a drug dealer, unfounded, insulting voice messages/texts) My question is, how far will this woman be able to go with police istance in invading this cousins life/privacy? And furthermore how should the cousin defend themselves against what is an obvious violation of the police officers code of conduct. The cousin usually does have marijuana in their home, and wants to know if just the suspicions of the one begrudged relative are sufficient to begin what could be a life altering and ugly legal battle?