I'm not certain of the etiquette, but since you have made a real effort to contact and communicate and there is no response, perhaps that is the response. It's up to you now as you are in possession of these life artifacts. Fortunately, as a creative and good-hearted soul, they have fallen into compassionate hands. I know you will treat them with respect.
The story of Casa Susanna shows our lives are full of contradictions. We all have secrets. The photos led to a quirky, beautiful true story unfolding because somebody found someone's discards and was touched. I'm sure you can trust yourself to use those slides artistically.

sounds like a question for randy cohen! and similar to a story from a couple months back in the sunday city or styles section of the nytimes about finding a diary in a dumpster....

Very true but that woman found the diarist and she was willing to speak with her!

I keep wondering if maybe my letter didn't get there, or maybe there is some underlying story that makes the family not want to respond, etc. I don't want to be pushy but at the same time I don't want to violate their privacy by using there past as my own drawing board.

I'd bet that they wouldn't mind as long as you weren't going all Mapplethorpe on the pictures.

My theory about these things is that if you are asking yourself whether or not you make one more try, you should make one more attempt.

I think I would want to know for sure that the family actually received my original letter of inquiry. Mail can occasionally go astray. Try one more time to contact them -- maybe just a copy of your original letter with a short cover letter explaining your attempt to re-contact. If they still don't respond, well, then it probably is okay.

I would suggest you speak to a copyright attorney, to avoid issues down the road.

Yes, speaking to a copyright attorney is a good idea. Perhaps send a letter again via certified mail. Keep copies of the sent letters to prove due diligence. Also, I guess it would depend on how long ago the photos were taken.... maybe they are no longer in the domain of the creator.

Yes there is the off chance that the letter didn't get to the person in the negatives, so if it makes you feel better...make one more attempt at contacting them and then you know for sure whether the negs are up for grabs.

Negs are never up for grabs.

From what I have read, the law is fuzzy about negatives that are found in public trash. The copyright of negatives (I think) lie in the owner solely but there are different laws that pertain to different types of discarded property. In this case, the owner is deceased, which adds another wrinkle, though I would suspect that the rights then transfer to the estate/family.

I'm not planning on profiting from the images, nor am I planning on publishing them in print or on the web at this time. I was just curious.

I might try to contact them one more time, just to check.

A filmmaker/artist in LA named Laura Daroca found a bunch of pix etc in a dumpster and made a movie about her quest to find the woman featured in most of them
Check this out: http://www.glossolalia.org/echo_daroca.php

Finders keepers. If you did your best to contact the person and still heard nothing, you're entitled to keep them. Especially if you're doing nothing for profit with the images.