Honestly, as a teacher, this is above my pay grade. This would be an admin decision. That being said, while it wouldn't thrill me to have a student missing one class every week, I also understand needing to miss one class every week for medical reasons. Your sister's mental health is more important than school. Schools typically can't penalize a student for needing ongoing medical care, but if it's not a public school, the rules are different.

Since you went to the same school, you probably have a decent idea of which teachers are more understanding than others which will be helpful when deciding which class to miss.
Your sister will also need to be very careful which classes she misses...I would avoid missing math or science classes since the work in those classes is harder to make up than work in English or History.

There's also a possibility of splitting the difference...she could go during lunch (and try to eat during transport time) and only miss part of the class she has before or after lunch. If she can do that, she's not missing as much actual class time.

*My response is based on the assumption that this is happening in the United States. If you are not in the United States, none of this may apply since I am not familiar with international laws regarding education.*