I also have a sleep walker-pee'r. My DS10 has done it since he was a small boy. His father also did it, although i thought he sleep walked and pee'd on things because he was a raving alcoholic. Since DS is only 10 i think it may be inherited. His father once peed in the food pantry. His mom was highly pissed! LOL no pun intended.
A few years ago DS got up at like midnight and i heard the front door open and shut (it was locked and dead bolted). he went out the door and out to the car, opened the car door and got in and shut it. strangest thing i ever saw. and he was sound asleep. Obviously i was lucky there were no dangers in our front yard and i heard him get up.
he's also pee'd in his toy box, heater vent, trash can. I've noticed he usually does it when he's in an unfamiliar place or OVER tired.
2 years ago we bought a house with an upstairs. Surprisingly DS has not fallen down the steps. HE seems to be awake and functioning (not running into walls) but when he talks you know he's sound asleep.
Not sure if i can offer any help except maybe limit her drinking before bed so she doesn't have to get up. THIS is the only reason DS gets up at night, cause he has to pee. then he just searches for a place.

God will forgive me. It's his job.
~~Heinrich Heine

Everyone in my family sleepwalks. My mom, dad, brother, my half sister, and I. When we were young, my mom took us to the doctors and I believe she was told it was inherited. It stopped being so frequent as we got older. My mother said it would happen out of the blue. I once pulled a tall dresser over and woke myself up. We have all sleptwalked outside at some point. I could go on and on with stories.

In your stepdaughters case, maybe she is getting herself so stressed before and after she visits BM and thats why its happening?

Make sure she is getting enough sleep, try not to let her watch tv or listen to the radio for bed (get her to read a book), and try stretching before bed to relax.

"If I turn into another, Dig me up from under what is covering the better part of me" -Incubus

I agree with DPW. I was a sleepwalker, but I only did it when I had something major going on. I really did it mostly during the school year. When I had Finals, report cards, etc. I would get really stressed because I was really competitive about grades. I have washed my hair, made cereal, gotten dressed and walked down stairs, all in my sleep.
I'm also concerned about her suddenly not wanted you to be in her room when she is changing. I know you don't do it and most likely respect her privacy but the fact that she is vocalizing it is alittle weird.
Try asking her is something is bothering her.

I was (am) a sleepwalker and so is my son. We usually do it when we are very tired or stressed. About 50% of our sleepwalking activities are because we need to go to the bathroom and we are dreaming that we are going, only to find out we've peed in a dresser drawer or corner of the closet or something. The others are just wandering. I once walked about a mile to the post office when I was about 8 and fell asleep on the steps out front. There is nothing really that my parents could do. They put a latch in the top of the door, but I'd just drag a chair over and undo it. My best advice would be to ensure she's going to bed early enough and make sure she's relaxed before she goes to bed.

My advice? After her visits from her mom's, I'd make her take a long hot bath, stretch, and put her to bed a bit earlier.

Good luck!!

Don't be concerned about her physical safety. Sleepwalkers have even safely driven cars only to wake up while underway. Yikes. Hide the car keys.

Its obviously stress related and likely to be outgrown. But how to keep the peeing under control. Have you Googled it? You need a speciality Forum or medical help such as a counsellor.

A chastity belt maybe?

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100 years from now it won't matter if you were successful in business, church or your social circle. But it will matter if you were important in the life of a child.