This...this is a Hell of a lot to take in, even for myself as an outsider for the situation. For you, it must feel insurmountable. I'm going to offer what I can...I wish so much I could simply give you the keys to this and have you and her stroll away into the sunset, but that is something I cannot do.

Understand, this may take a very long time for both of you...for you to be able to see past and for her to obtain healthy closure on, if that can be had. I'm not going to lie and tell you this is going to be an easy road. But all roads do have an end, no matter how far or how harsh.
I want you to think about yourself for a moment when you were a kid. Think about how you were introduced to sex as a concept. I'm going offer a very stereotypical assumption here, but I'm going to guess you got the standard "birds and bees" talk...you know, "when two people who love each other very much..." sort of thing. Your view on sex was shaped by how you were exposed to it. Parents and how they raise their children have an egregious impact on how their children form opinions on things, be it religion, politics, manners and morals, and in a remote way, sex.
Keep in mind the "healthy" way sex was introduced above. Now, imagine your wife's introduction to it. She never received the "birds and the bees" talk, never learned the mechanics from her idiot of a father (forgive me there, but I've zero tolerance for people who'll use religion to ruin someone's life. As a Christian, it sickens me that people can use something meant for such good to accomplish such horrific evil

), and never learned under what pretenses two people engage in such activities. Now, we introduce the older sister, who has the advantage of time. She has figured out, most likely, that physical stimuli "down there" feels good, and depending upon her age, has began the transformation into a woman, with all the things that entails. Your wife was introduced to sex under A) the notion that "what feels good is okay," which is in itself a common thing among children who're discovering their sexuality B) no constraints about what is proper and improper sexually and C) that if her sister and her can share this, then it must be okay among the siblings. It's a sad, sad thing to look at, but there is a logic behind it. Granted, it is a horribly wrong, horribly misguided logic, but it is there. Your wife's introduction to sex shaped, for those formative years of her life, her views on sex, and how she approached sex. It up and stopped about when I would expect it to...at 12-13, as you said, children develop away from stimuli based sexuality into a more adult notion of sexuality...not fully formed yet, of course, but enough to know that something "isn't right" about what's been going on. No one talks about it out of repression, and the relationship they have today is what it is out of a sort of mutual "burying" of these memories.
My point is this: Your wife, and the others, did what they did and suffered through what they did ultimately at the hands of a complete and total failure of parenting. Her father was an abusive, sick man, and her older sister was left with responsibilities unbecoming of her age and left alone to discover how to grow up. The result is this sad situation we see before us.

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As far as helping you two go, it's important you understand her perspective...I don't think your wife is to blame. If you understood my unerring, total hate that I feel toward anyone who is responsible for SA towards anyone, you'd understand that I do not say that lightly and without ample evidence that is able to convince me of vastly extenuating circumstances. I blame her father, ultimately...I trust he's out of the picture? I hope? Perhaps burning somewhere in the lowest pits of Hell? That would be nice. It's he who is to blame. I don't think your wife had the capacity to understand what she was doing. Now that she does...I can't imagine what she feels now. I think the distinction here is conscious choice. Conscious, informed, and understanding choice. Your wife did not have the capacity for that at her age and under her circumstances. Were I convinced she had such capability, I would consider your wife a monster, at best. With the information I'm working under, I see her as a victim of God awful circumstances wrought by the failure of her father and the abuse of her sister, and a survivor of those circumstances.
You must understand, it wasn't "real lust." I don't think she committed incest, in the sense of a conscious action. I think her level of sexuality was on par with a child who can be found with his or her hand down their pants at inappropriate times. It's not sexual for them. It just feels good. I think your wife was operating under the same basic, primal modality...the difference here is that she lacked the parental framework needed to keep her safe from things like this. I don't think she was committing a conscious, informed decision. I think now she understands how bad things really were...hence her crying, her shame, and her guilt now.
It's going to take time...I don't expect you to be able to simply digest this and move on, and I don't think she's going to expect you to either. I think this is something akin to her greatest inner demon, and I don't think she simply bears this as a small weight on her shoulders. I know you said resources are limited, but stick with whatever resources you have...for you, it may help to seek a therapist in order to work past your own feelings. For her, she needs someone who can professionally help her confront her hellish childhood and work past it.
I hope I was of some help. I would beg of you to PM me if I can do anything else for you...her past saddens me more than I can begin to tell you, and it pains me to see you suffer so.
Many hugs,
Harley