Sorry to hear that this hasn't blown over for you. I am really quite shocked that your younger sister would essentially blackmail you like this. As it stands now, I think you have a few different options open to you.
Option 1, you could find a new hiding spot for your stash or maybe even have your boyfriend take possession of your little things for a while, that is if he can take them off your hands without getting caught himself, this way if your sister keeps up her shenanigans you can rest easy knowing that she won't be able to locate your items and thus won't be able to make good on her threat of telling and showing your parents.
Option 2, you could purge your items. I don't usually recommend this, but desperate times call for desperate measures. I was forced to purge a nice stash of my own after my mother found it and responded poorly. If you really feel as though you might get caught and you know your parents will react in a negative fashion then simply get rid of your items, this way there is nothing around the house to incriminate you and there is zero chance of you being found out, your sisters threats would lose their power completely. If you have invested a ton of money into your current stash or have certain items that you are attached to, then I would once again advise against this option.
Option 3, You could call your sister on her bluff. Rule #1 when dealing with bullies is if they know they can effect you or get a rise out of you they will never stop tormenting you. One of the ways in which I fended off name-callers and the people who made fun of my nerdier interests in high school was to simply play their game, if they called me something I would say "yep, that's me", if they made fun of my interests I would act as calm and level headed as possible, I would usually say something like, "yeah, I'm into that, so what? What are you gonna do about it?". I found that if I reacted contrary to how they thought I would react they would just leave me alone. If you feel that your sister is too smart for the old "reverse-psychology", then I would advise against this, but she is rather young, I think you could match wits with her. If she flaunts the "I'll tell mom and dad" card again, just act the opposite of how she would expect you to react, she may lose interest if she can't hold this over you the way she thinks she can. Of course, if you decide to proceed with this option, it might be a good idea to hide your stash in a new place anyways, just in case you call her out and she does decide to get mom and dad.
In the meantime, try your best to stay calm, cool and collected. Option 1 and 2 might be your best choices. Ultimately, the decision is up to you. Whatever course of action you take next, I hope it works out for you.
Take care