Clever use of camouflage.

The morphing of the object is only affected by the timer, the few tests you ran in your room confirmed that. If you for example turn an apple into an orange for 20 minutes and then one minute later turn it into a banana for 10 minutes the effects will stack. It will stay a banana until minute 11 before getting back to the orange form until the 20 minutes mark.

You can use this, if you turn yourself into someone or something else for a short while you will come back to your sister's body and not yours, at least not for 51 more minutes.

You grab a dress on a rack and once wearing it you stand between two mannequins. You set the bracelet to the one on your left and 20 minutes before pressing the button.

Your are surprised to find that you can still see even without functioning eyes. The mall cops barge in and look everywhere, searching behind the racks and in changing room but finding no trace of the naked girl hiding around.

You would have to be careful when turning into inanimate objects as you can't move or communicate in any way.

The security guards leave the store quickly going to the next one.

You wait patiently, thinking of other ways to profit from this artifact or to mess with your sister.

The clothes around you give you an idea, you could turn any small item ( let say a piece of paper) and replace some of her clothes with it. With the correct timing she would think she wears her favorite clothes and on the mark they would disappear when morphing back. You could even switch her clothes from formal to slutty embarrassing her greatly at the worst time like during a job interview. Being back home you'll have access to her room and the preparation will only take a few seconds if you know what to do in advance.

On the 20 minute mark you can move again as you turn back into your sister. You ditch the dress, making her look like a crazy streaker is one thing but you don't want her charged with theft.

You run like hell out of the store, once in the hall you hear screams of shoppers calling the security. You manage to get out without being caught and run through the parking lot.

You can't take your car, it would give you away and besides you won't have the keys for another 29 minutes.

Squatting behind a large pickup truck you slip a finger or two into your pussy. Your orgasm during the video felt awesome and it was a good way to spend time before getting your body back. Running naked in public, even posing as your sister (especially posing as your sister?) made you horny.

You finger yourself vigorously, playing with your clitoris, enjoying the alien feeling of getting pleasure from body parts you don't have.

You have to change of hiding spot two times as cars leave the parking lot. You leave around damp spots behind you.

You finally cum, hiding behind an old van. The timer for your bag is now very short, you decide to use the van as cover for a little more.

You remove the rubber band from your wrist and on the mark it turns into a backpack. You put on your pants, shirts and shoes, not bothering with underwear but after wiping your pussy of its juices. You don't want to walk around with a wet spot in your pants.

A few minutes later you lose the boobs, grow a bit taller and looking back in the van's mirrors you see that you are back to yourself.

You walk slowly to your car like nothing happened and drive home.