The sisters death will be closely related to her secrets, so I think you need to discover what her secrets are first to understand how she died. Once thats determined you will know where to start your story from.
You could have a complete twist in the story. For example - and Im just throwing ideas around here - it could be not what you think. Maybe the girl killed her sister because she was jealous of her boyfriend as is trying to frame him? I always try to go by the advice that; you have a secret nobody else knows. In that respect keep your secret to the end but keep teasing audience with them. If you've ever read 'My sisters keeper' this is a great example of that, especially at the beginning when the narrator is contemplating murder.
Perhaps the younger sister already knows the secret, she knows who the killer is but by announcing it she has to admit her sisters own dark secrets [she robbed a bank, gave birth to a child, etc] She can't do this, so she tries to find the killer herself to cover the truth?
I don't know if any of this helps but I think once you know your story exactly you know where to start. Though If I had to suggest I would go by the age old advice, take us straight into the action, she's just found her sisters body dead.