Kids can express pain or anxiety by finding their room intolerable, either scary or lonely. It can go on a long time and it absolutely does not help for the parent to yell at the kid to go back to it. The room is too much, when a child cannot handle being alone with his or her fears.
Has she said what happened at her mom's (or whatever relative she was farmed out to) last time or the time before? I'd be inclined to believe something did. If you called the mom and asked, would the mom be amicable enough with you to tell you (if she knew)? Can she call her mom if she wants to or does the mom not want her to call? Was she sent to a relative on the weekend that the troubles began?
When I was working at a legal clinic, a little girl had been sent to visit her dad for a weekend, and when she came home she was terrified of taking a bath and of sleeping alone, when she never had been before. You guessed it, she had been sexually abused. I would worry big about this behavior change of your daughter if she is being casually sent around to other relations. If her mom picked her up and got some inkling that something bad had happened to her child, she might well not want to tell you, lest she herself get in trouble or lose visitation rights. So I'd tread really carefully in trying to find out what happened. You might even ask a professional family counselor to talk to your child, rather than you, so you avoid the claim that you somehow coached the child to say something bad about her mom. If you go this route, do it soon, so you can get past this night-terrors stage.
In the meantime, with no second comment about it, either let your daughter sleep in her brother's room (if he can tolerate it) or make up a bed on the sofa tonight for her. She deserves not to be frightened at night, on top of whatever else happened.
ps -- Cable TV in a child's room is not the best idea if you want them to sleep well. Research has shown that the blue light from screens (this includes computers) is disruptive to sleep patterns for kids if watched within an hour of bedtime. Be sure she doesn't watch it right before bed or it will disrupt her sleep on top of whatever happened.
It is not unusual for children to change patterns as they grow. I agree she is showing anxiety and loneliness. This is often seen in kids her age, even if both parents are in the house. While it could have been an experience that initiated it, it is not required. Many in her age group do not want to be alone. But it sounds like she is wanting her mom's connection that she is not getting. If you are not comfortable in doing so, please give her dad permission to cuddle and sleep with her on the couch in these insecure lonely times. If sleeping in her bothers room works, setup a bed in there for her. It is easy to let her share your bed, but this is not recommended and very hard to wean.
When she feels she can trust enough she will verbalize any scary experience.
I agree with Annie Brooke, if nothing comforts her make an appointment with a adolescent counselor. Even if there is nothing else involved they can help her be more comfortable with her mom situation.
Children suffer horribly if their mother abandons them. They never get over it - and she'll probably be in her 50's talking about how wonderful her mom was, she just didn't notice it and appreciate it while her mom was alive.
She's now old enough to realize her mother has abandoned her. We think you can give a child a loving, caring, beautifully clean and lovely home but they still want their mothers who are neglectful and irresponsible.
You can actually see, on an MRI, a difference in the brain of a child whose mother has abandoned her vs. a child whose mother is loving and attentive. The portions of the brain that deal with safety, food, and shelter are active in the abandoned children where same age peers with consistent loving mothers have the parts of the brain that are focused on higher level thinking lit up. Abandoned children are focused on staying secure, and can't afford to relax and use their brains imaginatively. So sad.
She's trying to exert control and power over her life, and she wants to decide for herself where to sleep.
I agree with letting her sleep where she wants to - with her brother, on the couch, etc. whatever works for her.
And expect her to spend the rest of her life trying to "make good" with a mother who doesn't really want her.