I can't imagine why you would be so upset that you would be working at trying to be calm.
Something happened. Maybe it's the nightlight, maybe it isn't. All kinds of things happen at these sleepovers - there's tension among the kids, this one can't sleep in a crowd, that one can't sleep in a bed that's different from her own, they can't admit they are nervous without their families, sometimes there's some teasing or scary stories, sometimes there's something worse (like inappropriate or menacing behavior by an older brother or the father - and no, you can't say "but they seem like such nice people").
The thing to do with a nervous tween or teen (or child, for that matter) is to BELIEVE THEM. She called, she needed to be removed from a situation, os that's what you do. You teach them that parents (or in this case, aunt/uncle) are a safe harbor, a place to call when there's trouble even if they can't explain it. You want a kid to be able to call when there are kids pooling or trading their meds, when there's too much beer, when there is sexual pressure, anything. You want them to call when someone is too drunk to drive.
First, you take this is a compliment. Next, you let her go to sleep with zero questions. You let her get up in the morning and have breakfast and calm down. Later, you gently probe and see if you can figure out what else was going on. You don't shame her by saying "This is all about a nightlight?" or blame the other family for not being able to figure out a solution.
I disagree with the comment below that the kids are too old for sleepovers! For many kids, this is the age when sleepovers first start, and for many kids they continue through high school.
What is clear is that these 2 kids aren't able to have a sleepover right now. So you move on to other activities that are safe and fun. If there's a request for another sleepover with the same kid, you have to sit down and brainstorm it. If there's a request for a sleepover with another kid, you brainstorm that one.
But nothing good will come of this if you can't calm down and if your husband is really ticked off that he was called to go get a kid at 1 AM. She won't trust you, and you'll never get to the bottom of it to find out the real cause because she won't see you as open to discussion or a safe port in the storm.