What laws are in place to protect a patrons information and check out history? Also related, what do these laws say regarding families and accessing or using each others accounts? Something along the lines of a husband picking up a wife's books that were on hold for her -- I realize some of this comes down to individual library policies.
In the United States, laws regarding privacy of library records are typically established at the state level. The American Library Association maintains a list of State Privacy Laws Regarding Library Records
Since you asked about Pennsylvania:
Pennsylvania Unconsolidated Statutes Chapter 24 Article IV § 428 states:
This is not to say that privacy of library records is not subject to other laws or regulations at the state or other level, not to mention the policies of individual libraries.
My understanding is that there are very, very few laws mandating patron privacy- and those would almost entirely be local.
Rather, it is a shared value of the profession, necessary in order to give patrons the confidence that they can seek any information they want (within reason and legality).
Also an official ALA position/policy: http://www.ala.org/Template.cfm?Section=interpretations&Template=/ContentManagement/ContentDisplay.cfm&ContentID=31883
Many states treat library privacy the same as medical privacy. You wouldn't want sensitive medical information left on your answering machine for anyone to hear, the same goes for your library books on hold. However, libraries don't necessarily enforce this consistently. If a husband comes to pick up his wife's books, it is often allowed. (Even though there are many tales of the husband picking up a divorce book for his wife!)
In California, the law was recently changed:
Interestingly enough, it allows tighter restrictions that go beyond simple check-out records (database searching, etc), but also allows written permission for another person (the husband in the above scenario).