Is it legal for a witness summons to be taped to the door of a family member that doesn’t reside in the same state as the witness?

A judicial summons is to be personally served on a person involved in a legal criminal proceeding. Legal action may be in progress against the person, or the person's presence as witness may be required. In the former case, the summons will typically announce to the person to whom it is directed that a legal proceeding has been started against that person, and that a case has been initiated in the issuing court. In some jurisdictions, it may be drafted in legal English difficult for the layman to understand, while several U.S. states expressly require summonses to be drafted in plain English and that they must start with this phrase: "Notice! You have been sued."

The summons announces a date by which the person or persons summoned must either appear in court, or respond in writing to the court or the opposing party or parties. The summons is the descendant of the writ of the common law. It replaces the former procedure in common-law countries by which the plaintiff actually had to ask the sheriff to arrest the defendant in order for the court to obtain personal jurisdiction in both criminal and civil actions.

The summons has to be personally served on the defendant. The family member is not required under the law to forward the summons to the criminal defendant.

Is the address where the summons was served the legal address of the out-of-state witness? If so, then yes, if it was served on an 18+ year old person who also resides at that address, it would have to be forwarded: it is good service to serve on an 18+ year old residing at a person's legal address. If it's not the witnesses legal address, then no--they don't have to forward it.