For the baby shower for my nephews (twins), they requested everyone bring diapers and wipes as well as another gift. Or maybe a box of diapers and a donation to a college fund?
Things a new baby always needs, even if there is an older sibling:
blankets and receiving blankets
bath towels and wash cloths
a double stroller, maybe from the group?
family membership to a children’s museum or zoo
books
lotions for baby and mom
onesies (they never last through multiple kids)