How do you blackmail your siblings?

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.