There should not be a "B" list for Wedding invitations. You decide how many people you can invite and send them out, usually about 4-6 weeks before the event. If you have lots of people who need lots of advanced warning, you send out a "save the date" card, then the invitations 4-6 weeks before the date. I think it is a little tacky to send out more invitations as regrets come in.
I would be hurt and offended too if I was your sister.