Okay, all that said, I generally only bring the invited kid to parties, and think it's rude to ask the host if it's okay to bring siblings because it puts the host in an awkward position. So my usual policy is, either arrange for childcare for the sibling, or politely decline the invitation.
Every other party invitation we've received has either been on a weekend (when DH can watch the sibling), or it's been someone we don't know well (a whole-class invite sort of thing) where we don't feel bad saying no.
But in this case, since the girl is a close friend of DD's, I'm hesitating to give up so easily. Should I:
-- Tell the mom that our RSVP is a "maybe" while I try to arrange childcare for DS (I'm worried that will sound like fishing for an invite for DS)
-- Ask if it's a drop-off party so that I can hang with DS while DD attends the party (it's within a mall, so there'd be plenty for DS and I to do, and I'd be right there to pick up DD when the party ended. But the girls are only 4/5, so dropping off isn't the norm yet and might make more work for the host)
-- Decline the invitation