Verdandi,
This is definitely an autistic thing - I've noticed that many autistic people are extremely protective of their personal space, and often view it as an extension of ourselves. I think this may be because the world outside that space is vast and confusing and overwhelming, but our space is reassuring because we have full control over it and know where everything is. If someone else enters that space and messes with it (or we perceive that they might have), that causes panic because of the perceived loss of control and invasion of privacy - it's almost like someone going into our heads and poking around. Maybe this will help you to understand your emotional reaction?
However, I think you are probably experiencing this very differently to the way your relative experienced it and the responses you have received are reflective of the way your post was worded. This will be particularly the case with neurotypical people who were raised in a home where people went in and out of one another's rooms all the time - it will be hard for them to understand the extent to which someone autistic could be protective of their personal space. While your relative probably understands that you don't want people to go into your room, to the average neurotypical this would be interpreted as, "I do not want anyone to go into my room without a reason". Your relative was probably thinking "I'm doing the dishes and I need that dirty plate from Verdandi's room, so I'll fetch it." To them, needing the plate to do the dishes is a perfectly legitimate reason to go into your room, and they would probably be very surprised to learn that you would not want anyone to go into your room even if they needed something which was inside it. To the average NT person this would appear to be excessive and irrational behaviour. Therefore, unless you have other evidence to think they would want to snoop in your room, please don't be upset with your relative - they just don't understand the extent of your feelings about it. I believe the fact that the other posters here are all horrified by this idea too is reflective of the fact that we are all autistic!
You are probably nervous about bringing it up because you are thinking that that would involve confrontation. Change the way you are approaching it - just be polite and express gratitude about them accommodating you, and you can make it something positive which can improve their understanding of you. You could say gently, "I noticed you went into my room for something the other day. I know it might seem weird or silly to you, but it upsets me when other people go into my personal space, so would you mind only doing that when I'm home? I'd really appreciate it. Thank you." If they know you are autistic explain that this is part of your autism. Also, you should be aware that a NT would read certain things into you putting a lock on your door yourself without saying anything, and they would be bad things! So if you want to do that, I suggest that you ask if it's OK for you to do that with the above as an explanation. This will make them feel involved (especially since it's not your house) and that you are not doing it because you are blaming them for something or being defensive but just to reassure yourself.
Sorry for the long post, but I completely understand where you are coming from because I have experienced similar. Hope it helps you.