So what did your sister do about it?
I just have to say that having worked in daycare for 10 years, it's a tough job, and it's easy to make mistakes. You'd like to think that something like that would never ever happen, but it does all the time. I've worked in super fancy daycares (set in the "Beverly Hills" type locations), and I've worked in daycares that were more in the 'hood, and I've worked in middle class areas (they seem to have it the most together). Wherever you go, however much you pay, there are always going to be issues and mistakes made. It's inevitable when you have so many kids to care for.
Teachers are always asked to go into other classrooms to give other teachers bathroom breaks, or lunch breaks or whatever. If a teacher is sick, her classes get split up and combined with other classes. Teachers get stuck with kids they don't know, Kindergarten teachers are suddenly stuck with kids who are barely potty trained, toddler teachers suddenly are stuck with 2-3 year olds who get bored with the toddler toys.... it's rough, but it's what happens all the time.
The director can't actually go into the classroom and take over, because someone has to be up front greeting anyone who comes into the building (for the safety of the kids, and simply to keep things running).
SO, as far as baby being given the wrong bottle.... it happens all the time. If the teacher was just in to give the regular teacher a bathroom break, it's easy to see how she could mix up the bottles. The fact that they told your sister about it means she's at a good daycare that really cares about her child's welfare. I have worked at several centers that wouldn't bother confessing such a mistake.
As far as being mad at someone or holding someone responsible..... it's really a combination of several people. The center director should have each of her teachers print out detailed directions to help any newcomers to the rooms. Plus, there should be pictures of each baby, with their name on it, so that anyone who comes into the room can figure out who each child is. The regular teacher should have given clear instructions to the sub teacher and made sure the sub teacher knew what she was doing before the regular teacher left the room. The bottle system in some centers can be complicated, but it's not rocket science. The sub teacher should have been able to figure it out and if she couldn't, she shouldn't have given a bottle at all.
So it's a toss up to say who is responsible. Calling the sub teacher 100% responsible says the director needed a scapegoat and treated that sub teacher very unfairly.
I don't know.... it's hard to say. I'm curious how it all turned out with your sister, though.
I just had to respond to share a former daycare teacher's perspective, so you can understand better how easily it can happen (plus, standing at my computer keeps my back turned to the mess in my house!).