I play both piano and guitar, and actually have been playing piano longer than guitar (I started with Piano in 2003). I had an instructor that taught me the basics, and so I wasn't really self taught but here are some general tips I would recommend.
1. you are absolutely right about being able to read music, this is a must with piano. The best way to learn do this is to start by playing scales, So that you see both the notes written out on a traditional music score, and also begin to be able to locate them on the fretboard.....errr keyboard I mean
2. I recommend staying with C Major scale to start off with, since it has no sharps or flats (Black Keys) and is a good way to learn the pattern of the major scale on the piano. I would then possilbly move to the relative minor of C major (A Minor) since it also has not sharps or flats, and helps to not only explain the difference between a major and minor scale, it also demonstrates the difference in sound between the two. When you feel you are ready to switch to a new key, I recommend G Major since it only had one sharp (F#) and introduces the concept of playing with both black and white keys gradually.
3. Don't worry about things like pentatonic scales, modes and exotic scales in piano. Things that we use every day in guitar are just not used in piano. For exaple, I never think about modes or pentatonics when playing piano, but rather the emphasis is on the diatonic (Seven Note) scale, chords, and playing within a certain key. So if you are looking to music theory with piano consentrate on the seven note scale, how to tell major scales from minor scales, know your keys and key signatures, your major and minor chords found within any particular scale, and chord inversions.
4. Are you going to use any kind of instruction book? if you do I recommend this one:
http://www.amazon.com/Adult-All-In-One-Cou...0471&sr=1-7It is an instruction book that is geared toward adults (How old is your sister?) , and teaches you quite a bit about theory, technique, and other aspects of playing piao in a user friendly manner. It is the first part of a three level course, and if you go through all three (I did) you will have a great foundation and be able to play quite a bit of stuff, and more importantly you will be able to pick up any piece of music and learn to play it.
5. If you are looking for music to play I recommend these sites, since they offer free music scores that you can use.
free-scores.com
8notes.com
freesheetmusic.net
That should be enough to get you started, but remember piano is just like guitar, practice makes perfect. Any other questions just PM me and If I can help you I will!