What happened with this little girl?

So my sister and I were babysitting 3 little girls tonight. The oldest one is turning 7 in august and the twins are turning 4 this month. Everything started of well. The parents left for a party and said goodbye. The kids were drawing and watching tv. We all played operation. Then, they had a little snack of lucky charms cereal and some water. We got the twins ready for bed, they brushed their teeth and went to the bathroom. Then we read them a few stories, and put them in bed on time. The mom asked us to put the twins to bed at 7:30, and we did. A few minutes later, she started crying. We went in her room and asked her what was wrong. She cried and said she wanted her mom to tuck her in. Well obviously her mom was not there. Her sister who's almost 7 helped calm her down, and she fell asleep. So we closed the door and that was that. The older girl was playing on the iPhone and playing tag after that. Then, we took her to the bathroom to use the toilet and brush her teeth. Then we read her a couple stories and put her to bed at 8:45.So the parents were still not home yet, so my sister and I just hung out and watched tv and played on our phones in the living room. At about 9:30, we heard very loud crying. The same twin who was crying earlier was crying again. But this time, a different type of crying. She was screaming, and it seemed like she was in pain or something. So we rushed to her room and asked her what was wrong. She didn't respond and kept crying. She shares a room with her sister who was sleeping, and I didn't want her sister to wake up. So I picked her up and carried her to the living room. I held her and rocked her and we tried to calm her down. We asked her if she had to go to the bathroom or if she was hungry or thirsty. She barely shook her head and kept crying. She didn't say a word the whole time. We asked her if she missed her mom and she didn't answer. I set her on the couch and gently stroked her to calm her down. I figured she would eventually stop crying but she didn't. We called her mother and had her talk to her. The mom talked to her on the phone but she wouldn't answer, just cried into the phone. So the mom called the grandparents and they came. The grandma held her but she still kept crying. After that they said we could go home and they would stay with them. I feel like I failed though even though we did everything we could. Do you think the little girl was just missing her mom? They've had various babysitters many times before. The mom said she does that sometimes and we just had to let her calm down. But she didn't. Maybe she had a nightmare? Or maybe she was sick or bumped her head or fell? Is there something my sister and I should have handled better? Was it our fault?

You must not worry! Very normal i think, you said the mother have told you it's not the first time.
but are you sure she was playing normally during the day? I mean this crying looks like a try to get attention (may be her mother leaves them long periods so she want her mum around) but things like that usually don't appear only at night.did you suggest them activities and games which is good for their age? (if you are a babysitter you will need some ideas of indoor active games, you can read about that in here:
http://firstchildhoodeducation.blogspot.com/2013/06/Indoor-active-games.html )
but although i think it wasn't your fault, you did what you had to do, so don't blame your self