There are 3 free (and DRM-free) "Inspirational" books from Harlequin's Steeple Hill Christian romance line on their promotional
Try Harlequin site.
Also, you might like to try loading up some of the public domain Christian "classics" like Pilgrim's Progress, although the writing style in those might be too antique for your sister's taste.
Or perhaps surprise her and pre-load a selection of books you think she might enjoy from our library here at MR, while giving her pointers to where to find the Christian fiction online.