Uh... I'm not sure if I'm the best person to give advice on topics like this (I've experienced too much death about me and handled it all wrong, for the most part).
All I can recommend is something that will take a very long time. However, I do hope that she has other people around her as well as you (merely for quantitative purposes, I'm not trying to say anything against you).
Giving her time and space is the first thing, because nothing is worse than forcing her to either forget it or talk about it. But, do this for as little as you believe possible, because 'giving her time' is more 'give her current mental state time'. This is necessary since people are in a bad way need a form of status quo if only for a while. Then, and I can't think of a different way of putting it, but very slowly insinuate yourself (as well as her friends, rest of the family) into her physical presence. Spend some time reading next to her. If and when she becomes accustomed to this, do something more obvious, like watch TV in her room (if possible), be on your laptop (or equivalent). After a while, step it up again and speak to her, but say things that at first cannot be answered. Then say things that can but not necessarily need to be answered. A warning, though, everything said and done must be as innocuous as possible because she will easily feel intimidated. Continue this, spend more time with her, steadily make things more social, even if she isn't responding, and human nature will eventually coax her into interaction with you.
The second major part is getting her to talk and do normal things again, even if on a small scale. Bring her snacks that she likes, comment about it. Bring her an activity that she enjoys, you can 'threaten' (toned down, and I mean seriously toned down) to take it away, if she says she's not interested. Do as she tells you. Don't bring her 'new' things all the time, only occasionally, and at some point, she'll ask a question. It'll be a relatively unrelated and inconsequential question, but that's what you're waiting for. Answer it and immediately ask her if she wants to do something that necessitates leaving her comfort zone (that might even be just having a meal with the family).
I hope you can see where I'm going with this, but as this continues, you're ever so gradually increasing the amount of social behaviour that she displays. I don't know how quickly or how effectively this would work in your case, but only that I had to go through it and it took more than three months (and this included professional help).
If in doubt, though, I'd raise it with your doctor (or her's if they're different) and ask only for a recommendation, as direct intervention may not end well.
I'm sorry that she's in this terrible place and I wish the both of you the best.
