JOEL - Your sister should wait until she is at least 19 to 21 years old and on her own before she decides to join. I assume your parents are agreeable to letting her now associate with members and friends of the LDS church? Young non-member teens can attend our church meetings and participate in just about any activity that the members do as long as their parents allow it. Some have even graduated from the youth Seminary program without being members. It's best to honor your parents wishes and be patient and wait for a better time, rather than risk causing any resentment or bitterness that could develop while she is still living with them. In the meantime, she can still pray, read her scriptures, continue to learn more about the church, and associate with her LDS friends.