I have twins, and when they are both invited to a party I spend twice the amount I would spend if only one was invited. (Approximately, since not all gifts are the same price). I'm not sure what I'd do for siblings that weren't friends with the birthday child (my children know all the kids in both classes at their school so they're friends with everyone even if they're not in their class). When I invite siblings to my kids' birthday party (or parties, if they do separate), sometimes I get cards from all the siblings, and sometimes not. I never stop to consider how much any gift costs because I don't care what people spend, so I have no idea if the names on the card reflects the cost of the gift or not. Honestly, anyone who dickers over the amount someone spends on a gift is not someone I want to be friends with, so I figure you're good to spend whatever you feel comfortable spending, and if the host is unhappy, then you've saved yourself from being friends with a yucky person.