I think there are a few ways you can do this.
1. You can pull her aside and explain to her what you expect from her as a player.
2. You can talk to the other players and have them talk to her about whats expected from her as a player.
3. Or you can use my personal favorite and just engage the group in a session of mostly ROLEplaying with very little ROLLplaying. Get the other characters involved and all of you engage each other in just great roleplaying. She will either get into it with everyone else or she wont. If she doesnt then you really need to see if her player type is good for your group or not. Being your sis-in-law it might be very hard for you to dismiss her from the group so I really hope that it really goes well for all of you guys.
On a side note my wife and I just moved to Hill AFB from Germany in late March and I was wondering if you knew of any good Gaming Stores. We are looking get involved with another group when I get back from Iraq after the first of the year. Hope my advice helps and Thanks for any help you can give me.