Hi! I believe I can be of help with this issue.
I can imagine how worrisome this situation must be for you. You are clearly a loving and caring mom and this is just so out of the norm. You are right to be both confused and very concerned.
The biggest problem and why there is confusion is that we just don't have enough information. It is unclear whether there is enough to warrant having a professional begin to inquire what might be going on or whether you want to keep an eye on it and do inquiries yourself. It's a judgment call at this time and up to you.
One of my supervisors when I was an intern, he's quite old now, used to say a wise thing:
With kids, one outburst that might indicate a problem going on leads to watchfulness; two outbursts lead to action. Her point was that we can't try to act as if we know what we don't know.
But you need to assess your level of worry about it. If it is high enough, then inquire from your daughter's preschool or from a clergyman who might be a good child psychologist in your area and call him/her to schedule a few sessions to find out if anything has occurred.
If you wish to handle the investigation on your own, then I recommend you do the following:
Make it a game as much as possible. Play body part games. Yes, she’s 4 and so I'm recommending being young with her. Make it age appropriate. Hair, nails, teeth, eyeballs, whole sorts of intricate parts. Get to the private parts and make them be something that's normal. Remind her they're normal and hug her and if she fixates, try to move on to the next item: toe fuzz or something. When she seems more comfortable with your inquiries and you're not so tense about it, go back up the body to private parts and ask if in playing husband and wife it involved THAT part? Just like you did with other parts. Etc.
I hope you are getting the idea: you are going to be tense and sending out whole sorts of emotional messages of worry and concern and that's going to influence any reactions you observe from her. So, calm down and be subtle.
Also, here are a couple classic books on body parts for kids you can use as props if you want. So use these books which you can get easily online or the library.
First Human Body Encyclopedia. It might be a bit young but so what! Amazon page:
The Magic Bus Inside The Human Body. Also a bit young. Amazon page:
Now the half brother playing husband/wife with a 4 year old is inappropriate. It's a very young game for an 11 year old in today's culture. I don't know his developmental status so it may be appropriate if he's very young developmentally. Your husband's explanation of active imagination is possible. You know if she's an imaginative child. But that doesn't change the half brother's inappropriate play. So, that would be something to discuss with her father both to gather more information about what's going on and to see if it can be monitored more closely.
Okay, I wish you the very best!
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