My girlfriends mum died earlier this morning what do I say to my girlfriend?

I was at work earlier today and lunchtime I checked my phone and I had 6 missed calls from my girlfriend. I didn't realise why she was calling me so much, because she knows I'll be working. Anyway so I called her back and she was in hysterics and couldn't speak properly to me, but she managed to tell me her mum had ped away early this morning. Her mum had been suffering from cancer and had to go into hospital, we all thought she was doing ok until that news came this morning that she had died. I don't know the full extent of her death and what happened. I haven't spoken to my girlfriend at all apart from this morning. I called her about 3 hours ago, but her sister answered and said that she wasn't feeling up to talking and that she was in a real bad way. I told her not to worry and to tell her i'm here for her and whenever she feels like talking to me, to let her know she can give me a call whenever. But what the hell do I say to her! I feel awful and It kills me to know that she is in such a bad state. I just want to hold her, but I can't it's really horrible. What does anyone say to someone after their parent dies!? She is the second oldest of three sisters, she is 19, her older sister is 22 and her younger sister is 11. I feel bad for all of them, but there isn't anything I can do.