Try out one of the other options.

You grab the calculator and have a look at the other options:

Merge - Combine two objects together

Morph - Transform an object into another.

Divide - You know what this one does.

Swap - Swaps two objects with each other.

You think about how your sister felt during her "session" which leads you to inspect the swap function on the calculator. You play around with it for a bit an figure out that there are different types of swaps that can be done, partial swapping allows for the swapping of body parts and full swap allows for complete body swaps. There are also other options that you are able to add on such as oblivious, normalize and aware. On further inspection you discover that adding the oblivious option makes it so the person swapped is completely unaware of the swap and will act as per normal despite changes to their body, the normalize swap is special in the way it can only be used by the person in possession of the calculator and it normalizes the situation to accommodate the swap making the swapped person act as if they were the person. Aware makes the person aware of the swap with all the panic that would ensue.

You decide on the normalize swap with your sister as the target. You put in the target and settings and hit the swap button. You black out...…..

You wake up a few moments later in what you recognize to be your sisters room you were in earlier. You look down and notice your sisters boobs on your chest and give them a quick fondle and relish in the feeling. You then take a quick look and your dick lying there on the bed, but also the calculator is sitting next to it.

"I guess that's what it means that the owner of the calculator can use it....." - you say with surprise as your sisters voice startles you.