Try this:

var stories = db.Query("SELECT NewsArticle.NewsId, NewsArticle.NewsDate, NewsArticle.NewsTitle, NewsArticle.NewsText, "
            +   "NewsArticle.NewsPictureUrl, NewsArticle.Newslink, "
            +   "NewsArticle.IsVisible, UserProfile.Membername, UserProfile.Firstname, "
            +   "UserProfile.Lastname, NewsCategory.CategoryName "
            +   "FROM  NewsArticle INNER JOIN UserProfile"
            +    "ON UserProfile.UserId = NewsArticle.UserId INNER JOIN Newscategory"
            +     "ON Newscategory.CategoryId = NewsArticle.CategoryId "
            +    "WHERE NewsArticle.Isvisible = 1 "
            +    "ORDER BY NewsArticle.Newsdate DESC");

Also check for typo errors... column name "Isvisible" = "IsVisible"?...