Dance shoes, UGH

Dance starts tomorrow afternoon. I had the girls try on their dance shoes today instead of waiting until we're at dance tomorrow and nothing fits. Rachel's ballet shoes are a size 1 and tight. I had her try on Sarah's size one's that she wore last year and there to big :( She wanted to go shopping and get another pair of shoes, but what size to get? A size 1?, That doesn't even make sense. She'll have to wear the tight ones for awhile without socks. Rachel's tap shoes were a size 13 and she couldn't get them on, I dug out Sarah's old pair of size 1's and they fit well. Then she tries on Sarah's old pair of size 1 jazz shoes and they're a big, but fit OK with socks. Why is there such a difference in shoe brands in the same size? UGH!