Where can I find this poem in the original Hebrew?
***
In the book _Hannah Senesh: Her Life and Diary, The First Complete
Edition_  (Foreword by Marge Piercy, Preface by Eitan Senesh.  Jewish
Lights ISBN 1-58023-212-4), there appears a poem on p299 called 'For
the Brothers.'  It was written in Hebrew in 1941 and appears here in a
translation by Ruth Finer Mintz.    The book itself does not indicate
a source for this poem.  Where can I find this poem in the original
Hebrew?

For The Brothers
    Hannah Senesh

Should we break,
Then take the burden
Heavy and great
Upon you.

Build upon sand
Under the blue
Sky ? everything
Anew.

And know, costly
The road to the just
And the true.

According to World Catalogue (WorldCat), the following Hebrew title
comes up when you do a search for "versions with the same title and
same author" as the one you cite ("Hannah Senesh: Her Life and
Diary"):  "Hanah Senesh: hayeha, shelihutah u-motah." Presumably,
then, this would be the same book in the original Hebrew. I found one
original (1945) copy of that title for sale ($20) at abebooks.com, a
secondhand booksellers network, published by Hotsaat Ha-Kibuts
Ha-Meuhad /6, Tel Aviv, 1945. Here is the page:

http://dogbert.abebooks.com/servlet/BookDetailsPL?bi=678565789&searchurl=sts%3Dt%26kn%3Dsenesh%252C%2Bhanah

Bookfinders.com lists this copy for sale along with another copy here:

http://www.bookfinder.com/search/?ac=sl&st=sl&qi=e9Ld3ztQHTJDNPXWHOa8RDpT5MU_6561660135_1:1:1

If you don't want to buy the book to get one poem, or you just want to
make sure the poem you want is in this original version, WorldCat
shows that copies of the book are held in a number of university
libraries throughout the US. If you have access to a university
library, you could probably get a copy through interlibrary loan, as
long as the copies are circulating copies and not in a special
collection. (Or, if you live in a city, there might be a copy in your
local university's collection.) Your local public library would likely
be able to get a copy for you through this avenue as well, although I
have no experience trying that route.

Dear Friend --
  Thank you so much for this comment.  I'm in New York, and will be
making a trip to the main branch research libraries, where they hold a
copy.  All I needed was this original hebrew title, as you suggested. 
Hearty thanks.  I hope I can return the favor (to you ... or to
another Google Questioner down the line)!
  ~matt