You can't be direct with her, if you break the secrecy you break the excitement. The thought of sneaking out while everyone is sleeping and doing something naughty is a huge turn-on for her.
What you need to do is make her incapable of resisting the urge to come to you, and in my opinion a great way of doing so would be working out shirtless every day.
Do not try to be obvious and do not mention it, just go out near your place where they can see you and get sweaty! Pushups, squats, crunches and pullups if you have a bar. Mom and sister will notice and start talking about you wich will make her crave you even more.
And rember, during the day she is nothing more than your neighbour!