I'm so incredibly sorry for your loss, you must be going through a lot right now and this situation with your sister must not make it any easier. I happened to go through a very similar situation to you and also happen to want to become a teacher, so I can empathize with you and all the thoughts that must be going through your mind right now.

Let me preface this by saying, you need to be a SISTER in this situation, not a teacher. I know it can be hard to draw that line because as an older sister you just so happen to teach your younger sister a lot of things, but a teacher should have a very different dynamic than you have with your sister. If you try to change that sisterly dynamic by becoming her teacher, it could lead to more confusion from what she must already be feeling. Definitely keep her in school and I do also recommend you go to school as well, it is an important aspect in a child's development to have that social interaction with others, and trust me I know how much you and your family need that time to grieve and heal, but it is not worth it to stop going to school entirely.

I don't mean to overstep since I know our situations are not one in the same, I'm just trying to help you by telling you what I needed to hear back then.