Whether its frowned upon to have a shower for the 4th & 5th, depends on the situation. I think it is okay, since they are strapped financially, and probably need things.
As for what to get her, I would lean towards a group gift of a crib or the car seats, since she will NEED those. A ton of diapers and wipes, etc.
As for her attitude, she's probably expecting mom and some other sister to buy her all the big new expensive stuff like she received before. By you trying to do a group gift, you're taking away her ability to play the heart strings while out at the baby store and come home with a truck full of stuff.
I've been in a similar situation. The mom-to-be, was hoping that the family would all buy her the big new expensive crib, mattress, travel system and changer that she wanted, instead of us doing a group gift of just the crib and mattress or just the travel system.
If you really want to stick it to her, just buy her a Diaper Champ for her crappy attitude. ;-)