I cannot remember the name of this book/series?

When I was in high school (about 7 or 8 years ago) I read a series of Christian books. They were for young adults/teens and I remember that they were about a brother and sister. Their dad had gone missing on a missions trip I think and when they had trouble, they chatted online with this man who gave them advise (I never finished the series, but I had always assumed that this man would be revealed to be their dad). The one I remember the best was that a new-age book store had opened in town and the teens' mom was gone for the weekend. The lady at the book store asked for their help because she knew they were christians to deal with a house that had demon activity. I think several of the books deal with spiritual warfare, but I cannot remember any of the titles, the name of the series, or the author. Any help would be appreciated.