It's nearly Christmas so I hate to be uncharitable but in the case of ole abe I'll make an exception
I think the story you are referring to is most probably that of "Mrs Bixby's five sons".
According to the story a Mrs Bixby of Massachusetts had 5 sons killed in action in the WBTS and ole abe himself sent a heartfelt personal letter to her.
As it turns out this like most of the lincoln myth was a fraud. The letter was probably forged by John Hay and printed in a Boston Newspaper for public relations/propaganda purposes.
This is a good example of the way historical frauds are perpetuated by the well the meaning but ignorant. "Mrs. Bixby's letter" was mentioned in the recent movie "Saving Private Ryan". Kinda makes you wonder what else you've been lied to about don't it?
http://www.trutv.com/library/crime/criminal_mind/scams/
lincoln_forgers/
http://symonsez.wordpress.com/2009/11/21/large-hadron-
collider-up-and-running-no-black-hole-yet-mrs-bixbys-
letter-to-lincoln/
scroll down to "on this date in history" on this one
The US military does have a policy, dating back to WWII, that allows surviving brothers from families that have lost 2 or more sons to be assigned non-combat duties upon request.
http://ngm.nationalgeographic.com/ngm/0106/feature5/
index.html 
