How can I prevent (or catch) my mother and sister from snooping through my things when they come over?

You could use a small piece of scotch tape on things that open to see if they looked in. I would be pissed and would probably confront them about it. Or you could go with this.

Mouse traps.

Mis-information. If you’re willing to spend the time, write a fake letter, or several, and hide them where they will look. Perhaps make it very adult and shocking. Or annoying, but harmless booby-traps, perhaps involving permanent red ink. That way you can catch them red-handed.

Close your room door and put a sign that says “Trespassers will get electrocuted” or if you want something lighter “Trespassers will go to bed without dessert”. The day they will be home without you, set up a camera to record the room (handycam/webcam will work).
The mature thing of course would be to talk to them about it and tell them you aren’t comfortable with them looking through your room since its an invasion of privacy. Other ideas include tit for tat by looking through their luggage, finding anything remotely embarrassing and parading it in front of their faces (this plan not recommended unless you have no other option), you could also tell them that if they can’t respect your privacy they can stay in a hotel for the time they are in town (money is always a good motivator). I can’t believe your family does this, I can imagine how upset you must be about it.

PS – Oh and as for the whole “see each others purses to bond” thing, I recommend creating your own rule, “see something you like in someone’s purse, then it’s yours!” rule. Tell them this rule is essential for you to bond and feel that you trust each other with your belongings. You will have to do some prior planning of course and make sure you don’t leave any money or anything appealing in your purse so they use the rule against you.

I would pack up every single personal item and put it in a locked place off-site (storage unit for apartment, car trunk, etc.)

Despite what they are telling you, they are not entitled to do any of that snooping. They just have no boundaries. Just because they have no boundaries, does not mean that you are not entitled to yours. You know, however, that they will not respect your wishes in this matter, so you need to take the step above.

I would never let anyone go through my purse or home. After this occasion, I would tell them that if they cannot respect your boundaries, they are not welcome in your home. You will either see them at their homes or meet them in a public place.

As to your neice (or your sister or Mom) and the purse, I would say “No, you may not go through my purse. It contains personal things. Here, I brought you this to play with (a toy purse stuffed with things).”

Do not engage with them or argue. You won’t change their minds. Just say, “Our feelings are different about this subject. I respect your right to your feelings, and I expect you to respect mine.” If they persist, just keep repeating the sentence. Do not get into why.

great idea, if they go through medicine cabinets, empty the cabinet out, open it up, put a piece of cardboard in front, fill it with marbles/bouncyballs/any thing that will produce a good result, the cardboard is there to keep the balls in temporarily. then shut the cabinet door, slide out the cardboard, and wait till they open it up. slight surprise, and its pretty easy to catch her with this one

I would say just place something very shocking or involving them that is fake. But something that they will have to confront you with. That way its something that dosent matter and you can find out the truth.

Small pieces of duct-tape on the side of drawers/closets ect. It has to be so thin that it’ll snap without the person opening it notices. Then you can see where they’ve been.

If you don’t want to confront them, or it doesn’t work anyway. Try pulling some joke at them. Make a fake document saying you’re a member of a cult or something, so when they find it in a shelf or anything, they’ll be shocked. Then you tell them the truth and that you don’t want them sniffing around anymore

In the James Bond books he puts small pieces of string or hair or stuff like that on doors and drawers and the like and checks to see if they’ve been moved, indicating that someone had opened them. Might be an easy way to keep tabs.

Hey your tag here is Tenacious D. You should be tenacious. Take charge, darlin. Padlocks are the answer. Marina’s right as always: talking isn’t going to accomplish anything because they don’t care what you think. Take action.
Afraid they’ll be OFFENDED? oh sorry, shouting. Why? they don’t care if you are.
You may
love them
but
they’re uncivilized. You have to become
alpha with these, sorry, dogs of yours.

write them a note about how you know they’re doing this and what the meaning of boundries happens to be.

Wow. There’s a lot of mission impossible stuff suggested here- why don’t you just talk to them. It’s your house, right? Why back down. They are your family, so hopefully they would be able to listen to your feelings and respect your wishes. It’s not like they’re goign through your things secretly or behind your back. So when you watch them doing it, just say “Hey, I would really appreciate it if you didn’t go through my stuff every time you come over. It’s kind of rude to me.” Or something, I don’t know.

If you have to go to work and you don’t feel comfortable leaving them in your house, then tell them! All this passive aggressive crap will just make everyone involved unhappy. Including you.

or, dont let them come over?