Firstly.....I find it kinda strange that passion is hidden in the depths of the earth! I find that passion is sooooooooo much closer to the surface than that!
Gman, what more CAN you do? You have made it clear how you stay. If we're talking "dear Abby", then I must ask you how much you are flattered by this passion? We are all human, after all, and do love to loved, even if we don't return it to the same degree. Men and women have longed to be worshipped (Godlike), since the beginning of time, and possibly this human flaw is driving you?
Do you know of the phenomenon called "transference" and "countertransference"? Check it out, and then see if somehow you are linked into this sister in that way.
Sorry to get all "psychiatric" on you, but that's how I stay!
If you can truly answer "No" to all these things, then the sister will have to work out her own problems, you have done your honest part.
Naphti