Leaving aside the fact that "blackmail" is a criminal offense in its own right, and you should report it to law enforcement, if that is going to be your defense you had better be able to produce some convincing evidence of it because you could be charged criminally with 'Obstruction of Justice' or Contempt of Court.
Also:
It is not contempt of court if you are being forced to withhold the evidence against your will as long as you would, absent the blackmail, have provided the evidence. To be in contempt of court, a person must intentionally of his/her own free will, refuse to do that which the court requires. The element of blackmail negates the required intent and willfulness.
Blackmail is a situation in which you agree not to share some piece of information in exchange for money or cooperation. If you have a crime to report and go straight to the police with it, you will not be blackmailing anybody. If you are being blackmailed into withholding evidence of a crime, you can't prevent it from being blackmail. You either have to cooperate or report the crime and ask the police to protect you.
It's not very sensible to blackmail your siblings (depending on the seriousness of the blackmail). Emotional blackmail can be very harsh and therefore could make you and your siblings drift apart or create a rift between you. However, if you must blackmail them, then it is best to find some information that they really don't want your parents to find out. Then you threaten to tell your parents the information unless they give you whatever it is you want. If the matter is very serious, then blackmail isn't a good idea as it is illegal.
Violation of ethics
no because some people don't know what withholding is
Another name for blackmail is extortion. It involves threatening to reveal information about someone unless that person meets certain demands.
by hidding the information
Violation of ethics
no
Depending on the context, blackmail is already a verb. For example "to blackmail someone" is an action and therefore a verb.
Gatekeeping
It is illegal.
blackmail