5 year old wants to sleep over then cries!

Why does my 5 year old niece beg to sleep over but then wakes up in the middle of the night crying unconsolably?


Some background:  i have a 4 yr old daughter, they play well together despite the fact that my niece is a bit jealous of her.  Only because my niece loooooves me. We are very close and i treat her as if she were my own.  My sister (her mom) broke her leg recently and i had to care for them recently, she had to spend the night while momwas in surgery.  She has spent a night before but didnt want to again because she has to sleep in my daughters room (she usually sleeps with in mom and dads bed and her baby brother). Lately shes been begging to spend the night but then wakes up either screaming uncontrollably or cries silently, whimpering in my daughters bed. No night terrors, she just says shes scared.  If shes scared i would think she wouldnt want to come over anymore but she begs and begs to come!  Why?? I dont get it.