So she's hurt. It may sound cold, but she's not your sister. It doesn't sound like she's even your stepsister any more. You see her so infrequently that she has to tell you through other people that she's hurt. So overall, this has very little impact on your life.
You can't control the actions/feelings of those around you and that includes forcing someone into what amounts to a very special intimate family relationship. You don't have those feelings of closeness toward her. Her feelings are hers to deal with. If your other relatives continue to ask you about the situation, just tell them that you are comfortable with the status of your relationship now. If she continues to make inappropriate* requests on facebook, defriend her.
I struggle with something a little like this from my SIL, who tries to force me into a sisterly relationship with her, which in her mind means there are no boundaries between us, I give her whatever she wants, and she gets to unload her problems/burdens/untold TMI on me. Her constant push for more closeness to the point of some really inappropriate behavior on her part, lead to me totally shutting down around her emotionally long before we cut off contact for other reasons. I have a sister. I know how special that bond is and that bond is not something I'm interested in sharing with SIL. And if someone pushes that too much, it will blow up in their faces. Instead of having a reasonable, cordial relationship with me, she has no relationship at all.
*inappropriate in that they are making the OP uncomfortable by attempting to move their relationship beyond the OP's boundaries