That depends on a number of factors. Firstly, if your sister will play the game with you. I’d say the likelihood would be high for this. That means she will be in the medium with you.

I have never heard of two dreamers sharing the same tower on their moon, however that is not to say it is impossible. I find it more likely that your sister would be in her own copy of your room in another tower, but it is still possible that you will share a tower.

If the two of you are sharing a tower, its likely that you would share other things in the game as well- perhaps titles, lands, quest beds, or any combination thereof.