Welcome to the forums

It is great that you have such a good relationship with your sister and that she is there to support you. It's always up to you who you tell, and only when you feel comfortable, and it can be extremely hard to have that conversation with family members. From your post it seems that your sister knows about your past self-injury. If you don't mind answering, how did she react last time she found out about it? It can often be a good judge of how she would react now.

Welcome again. I hope you can find what you are looking for here. It is a great place to get support.