A very very soft YTA.

Sally 100% should have stepped up in your time of need. But, it's not completely wrong to say that Sally had prior plans and you know she doesn't babysit, especially as it was so last minute. Yes obviously no one plans an emergency, so I absolutely do not blame your for asking. But she said no, she couldn't do it last minute.

If Sally had asked you to make the cake and you said no immediately, you'd definitely not be the AH because you're right, her wedding isn't your problem. But you've already agreed and you're going back on that. It can be difficult to find a bakery to make a fancy wedding cake and this is something Sally thought she'd solved (having a cake), but now she has to go back to square one. You don't mention if you're getting paid to make it, but if you are then it's technically a customer agreement and you should be making it.

Again, if you said no immediately, like Sally did to babysitting (which again I don't agree with, she should've stepped up but it's her choice) then you'd be fine. Since you've already agreed, make the cake, then cut contact with her if you want to. If you don't make it again your choice, but be prepared to have not only your sister, but potentially other relatives upset with you.