If there is no proof of the burglary and of the theft, and the item is not found, I don’t see how it could be a chargeable offense. It would be a case of “he said, she said.” If this seems likely to happen again, I would put a camera in my room, discreetly of course.

Definition of buglary: ”entry into a building illegally with intent to commit a crime, especially theft.”

Since they can already enter the building without intending to commit a crime, no, it’s not burglary. It could still be theft.

What’s missing?
How do you know that the sibling overcame the lock, got into the room, took that thing?
Seems to me that entering a locked room is some kind of invasion of privacy, taking something adds burglary/theft.
The parents should have something to say about boundaries in the household, and what rights and consequences, etc. A court of law might listen to what they have to say on the matter if charges are made.

Hmmm, @Zaku, what if it’s a detached building like a garage or a storage shed? Say one keeps stuff in there and the other is accused of getting past the lock.

Does a theft have to have a certain value to be treated as a crime? Sure, a bicycle maybe, or a computer, but what about a set of calligraphy pens or a folding knife? What about a spare car key?—which of course means access to the car and not just the item itself.

I have never really understood how you recover a theft anyway unless the culprit is caught with it or there’s a trail and witnesses. If I claim my piggy bank is gone and it had $300 in it, how is that anything but a claim? I can point to the spot where the piggy bank isn’t, and even show a photo that it was there, but the empty spot proves nothing. The claim of an amount proves nothing. Sure, a museum loses a Rembrandt, and everybody knows it was there, but who knows if I really had $300 in a piggy bank, or for that matter a sapphire and diamond necklace in a jewel case? All you’ve got is the empty spot. How would you ever claim it back? There might be records of an expensive necklace, but not of petty cash or some CDs or a computer tablet.

And so what if it’s just one sibling’s word against another, what are the parents going to do, or a court of law?

You can report stuff missing, to the police, especially if it includes a car key…
Whether they will do anything about what you report will depend on their availability for that kind and level ($$ value) of the theft The car key, and information about the car of course, could help police follow up later if the culprit messes with the car.

…what if it’s a detached building like a garage or a storage shed? Say one keeps stuff in there and the other is accused of getting past the lock.
– For buglary (only), it would depend on whether entering the building is provably done with illegal intent.

Does a theft have to have a certain value to be treated as a crime?
– Laws vary by place (jurisdiction). In general, usually no value is required for theft, but in some jurisdictions, stealing valuable or particular things may be a more serious crime. So can stealing things in specific circumstances. In many cases in the USA, theft of something under $500 value is “petty theft”, and may only be a misdemeanor, rather than a felony/crime.

@kneesox It is stealing to take something that doesn’t belong to you.

@si3tech, yes, sure. But the Q is about whether it’s going to be treated as a crime. And what happens if the supposed victim claims that it IS a crime.

The night cleaning staff’s kid stole a trinket off my office desk (so I believed, anyway), and it was theft, but nobody’s going to prosecute that. It’s just a trivial loss that you have to take in stride.

Even in that case though, how can you ever prove something is missing? It’s obviously not there now—but was it? It’s not like a dead body, which IS there and proves that something really happened. What if somebody just imagines that the other person stole something? It looks the same as if they really did.

Do you have proof?

Was the object found in the person’s possession?

Are there any witnesses?

Other than that, it’s just speculation.

Even in that case though, how can you ever prove something is missing? It’s obviously not there now—but was it?

In the eyes of the police, I would guess that it comes, at least in part, to motivation on the part of the victim. Why would they make it up? Can they describe the object in detail? Do they have an ulterior motive for accusing the suspect? ... etc.

B & E anyway.

Petty theft is still theft, and can have serious consequences. Even if you just report it to a cleaner’s employer, they might lose their job or suffer a worse relationship with their employer.

If you really can’t prove it, you should really make sure you know what’s going on, so you avoid doing serious harm to an innocent person.

In the case of a trinket that’s no longer there, and seems to have disappeared when you were not there but other people were, you need to consider how many people had access, and whether it’s possible it got bumped or mistaken for trash or whatever.

Consider that a cleaning person probably knows their job would be at risk if they take things, so how likely does it seem they would risk that to have a trinket?

You can try asking them about it from the perspective assuming they did NOT take it, and you’re trying to figure out where it is, e,g, “Did you happen to notice if my [description] was [where you remember it] when you cleaned the room yesterday?”

If you’re pretty sure someone has taken your thing(s), and want to prove it, you can try finding out, for example by leaving something else you expect they’d take, and checking right before and after they are the only one there.

Or set up a hidden stealth camera such as one made to look like a book, etc. and have it pointed at your desk.

Thanks for all comments. The item taken from the desk was years ago and was just meant as an example of a petty theft that no one would go after as a crime. The original question was about criminal charges for entering a locked room in the same house.

@kneesox A stealth camera in that locked room would probably offer you the only information that a crime had occurred.

Trespass

If the siblings are not living in the same house, it is theft. If they are living in the same house and both of them are adults, it still is a theft because the sibling has technically broken in by picking the lock. If they are living in the same house and are not yet adults….I’ll check with the parents first. They’ll know the truth.