Sorry to hear about the situation, Tears.  That sounds awful.  I would google how to get someone to eat, but here's are a few random ideas I had -- not sure if they would work but just things I thought of.

1) Cook her favorite dishes.  If you don't know how to make them, order them from a restaurant.  Do it in a romantic way (set the table with candles?  use the good dishes?  bring home flowers as a centerpiece?)  She may only have the appetite to nibble a small amount, but you are showing her you care, and with all that effort put into it, you'd think she would try to stomach at least some of it (unless she has an eating disorder like anorexia).
2) Buy those protein type shakes -- you know, those meal substitutes drinks (like Ensure) for people who can't eat and need their nutrients.  It may be easier for her to drink something than eat.  Buy a daily multivitamin as well.
3) Using a juicer might be also be good.  When I'm not hungry, I'm amazed how many vegetables I can eat if they're served in the form of a blended drink compared to if I had to lift a fork to my mouth and eat.
4) The above are based on if she knows that she's not eating enough and her lack of appetite is due to stress.  If you are encountering denial/rebellion that she's not eating enough, then she's on the defensive.  Definitely don't nag in that case.  Don't make her feel like she's doing something wrong -- don't criticize that she's not eating enough.  Instead, I would just pamper her and show how much you love and care and are concerned.  So in other words, not saying, "You should be eating more" (because the word "should" might sound like criticism to her, like she's doing something wrong), and more like, "I'm worried that you're not getting enough nutrients lately.  I care so much about you and your health and I'm here for whatever you need..."  

If you suspect anything like anorexia, I would also google "anorexia" and see what they suggest for that (they probably suggest counselling).  But for a lack of appetite just due to stress, I'd go with pampering her with all her favorites, especially lighter foods that are easier to digest... and even if all she craves is junk food, in small doses, it's better than no food at all (at least in the short term).  Just make food and drink available and put a lot of thought and care into how you present so she's touched by your efforts instead of annoyed or feeling manipulated.  Remember how stressed out she is and how to her mind, feeding herself doesn't feel like a priority.  

If you can't cook, talk to someone else who loves her (like a family member) and maybe they would bake something special and bring it around the house.  

Good luck with everything.

Given what she's going through, is there possibly a grief group she can talk to?  People who are going through the same thing might make it easier for her - or a grief counselor could.  She sounds like she may need more help than you are capable of giving alone.

I agree with caircair. I think finding a grief counselor would be a good move at this point. You can offer to go with her the first time if it would make her feel more comfortable. But at this point she's mourning the loss of both parents, and that's more than you can handle alone, as much as you want to help and fix everything for her.
In terms of eating, I think woundedpuppy's suggestion of shakes is a good one. The fact is, she has to eat even if she's not hungry, and little sips of something throughout the day sounds like a doable compromise. Don't get into an argument over it, but just tell her that you care about her and need to see her taking care of herself. You have to take care of yourself too, remember.
A quick google turned up this site, which seems like a good one.
http://www.tchospice.org/grief-reactions

But I think the first, most important thing you can do is talk to a grief counselor. Try to get her to go, and if she won't, go yourself.

today was very difficult......I had been to see a grief counsellor yesterday on my own, to get some help and some ways to get the pair of us through this, and so today i sat down with my girlfriend, and told her where i had been and what she had said. She didn't react well, and thought that i couldnt deal with her anymore and that i was looking for a way out...i was stunned, i told her it had all been for her, and that i loved her more than life itself, and that i just wanted to help her get through this....we had a big argument, and everything that has happened over past few months just seemed to all come out in this one big blow out....she asked me to leave after it all, and so i did. However nearly 2 hours later, i got a phone call from her, asking me to go back and see her.....i did. When i got there, she was in pieces. Tissues surrounded her, tears still streaming from her face, when i got in, she threw herself at me, and wanted me to hold her, so i did...she then kept apolgising for her reaction, and i told her it was ok, and i understood, its a very hard time for her at the moment, and i just held her in my arms for a while, whilst she cried into my arms.

things are very difficult right now, and im struggling to cope...

I'm sorry you had such a hard day. I'm sure she's just so overwhelmed that all of her emotions are coming out in all kinds of crazy ways. I'm so glad you went to see the grief counselor, and I think that will really help in the long run. Keep doing what you're doing; you're a fantastic boyfriend to her.