I give gifts for everything! Don't get me wrong, they are modest, usually homemade but always from the heart and personalized.
Here is what I do in this situation...I make tee shirts for the older siblings and give that as the gift. Do this with a computer and printer iron-on or with craft paints. I personalize them if I know the newest arrivals name in advance, such as "I am Michael's BIG BROTHER!"
Once I did this for a family of six siblings...it went "I'm the BIG BROTHER", "I'm the BIGGER BROTHER", I'm the BIGGEST BROTHER" etc.
Tee shirts are cheap to send in the mail and many of my friends have been tickled pink when their kids wear them to the hospital to meet the sibling.
If money is truly tight, who doesn't appreciate heart felt words of congratulations and encouragement? By all means send a card!