Can I legally share love letters over 15 years old?

Yes, they are legally your property. You can do with them what you think is best.

Yes, absolutely. You didn’t sign any kind of document that said, “You may not share these letters with anyone, they are confidential and for your eyes only”. As @WestRiverrat said, you own them.

Absolutely. You have nothing to worry about.

I believe that according to law, letters remain the property of the author. This is so at least for copyright purposes, if I am not mistaken.

Two words if you get worried: plausible deniability.

”What letters? She found those, oh my, I’ll have to ground her for a month.”

Ah, it looks like @Jeruba could be right. Everything I find online seems to state that whoever wrote the letter has copyright over the words, until they are deceased. That said, I’ve so far been unable to find any actual laws that clear this up.

Still not an actual law, but this is interesting:

“Allegations involving copyright infringement frequently occur when the author of an unauthorized biography makes use of the subject’s published or unpublished letters and papers or possibly from oral conversations the author may have had with the subject.

In Salinger v. Random House, Inc., the author’s use of extensive quotations from unpublished letters written by J.D. Salinger, the subject of the biography, without Salinger’s permission was deemed to be copyright infringement. Under copyright law the writer of unpublished letters has the right to control the first publication of those letters.

Criticism of the Salinger decision as well as other copyright infringement decisions based upon what was believed to be excessive protection for unpublished works, resulted in Congress amending the Copyright Act. U.S. copyright law now provides that if a work is unpublished and it is used without permission of its author then the fact that the work was unpublished by itself would not be sufficient to constitute copyright infringement.

Subsequently, in Wright v. Warner Books it was held that when a work is one of “criticism, scholarship or research,” that quotations from unpublished personal letters and journals might constitute fair use.

In Estate of Hemingway v. Random House, Inc., which concerned the publication of portions of conversations between the author and Ernest Hemingway, the court ruled that the author of Papa Hemingway did not infringe the common law copyright of these conversations by including them in a book on Hemingway’s life. The court stated that even if the author used verbatim some of Hemingway’s words that such use was minimal and qualified as fair use of the material.”

From here

You’re not publishing them, you’re letting someone look at the originals. No copyright laws are involved.

You’re just showing them to his daughter, not publishing them. Unless they’re going to be read by the public, there’s no legal problem there.

The letters are your property. You can share them with whoever you like from the moment you open them if you like, and the sender could be subject to legal penalties if he tried to take them back from you. The content of the letters, however, is owned by the sender. You do not have the right to republish them without permission unless you are operating within the scope of fair use laws.

E-mail is a special case that has not yet been tested. A series of seminars I attended on copyright law insisted that messages are out of our control once we hit “send.” Forwarding is so central to the institution of e-mailing that no court is likely to outlaw it. Most legal experts predict that forwarding will be considered the equivalent of showing a letter to another person in the digital world. Moreover, the basic functionality of e-mail already requires the creation of several copies for delivery. This means that a court would have to outlaw e-mail altogether if no copies could be made.

There are intermediate positions available, such as outlawing unauthorized copies and declaring those necessary for delivery as implicitly authorized. But I accept the predictions of legal scholars I know who say that any case regarding e-mail will probably declare forwarding to be fair use unless it meaningfully infringes on the rights that copyright law is actually meant to protect. If someone forwarded a poem or story that was transmitted for review via e-mail, then the message might be protected. But in most cases, the nature of an e-mail is such that it has no real market value except insofar as it contains information protected by other laws (e.g., patent or trade secret laws).

I defer to @SavoirFaire‘s distinction between the physical letters and the rights of ownership of the words that the letters contain.

Defer means “to postpone”. Do you mean “Refer”?

No, I don’t.

defer
–verb (used without object)
1. to yield respectfully in judgment or opinion (usually fol. by to ): We all defer to him in these matters.
[ source ]

English is confusing. And it’s the only language I speak.

Nothing is going to happen to you legally if someone reads the letters. You didn’t sign a contract saying you’d keep them to yourself. Just don’t publish them and you’re fine.