Aita for wanting half of the Rent

It’s about the following: A few months ago, the topic came up that my sister might want to move to the city where I live to open a shop there. Some background: A few years ago, she lived with me for a short time and only paid the utility costs, since we also shared a room and she basically didn’t have her own space because my third room was sublet.

Last year, I then lived with her under the same conditions. We shared a room, and I paid her the same amount she had paid me back then.

Now the topic has come up again that she wants to move to my city. My third room is free, and she could have her own space. I now want half of the total rent (warm rent). I haven’t even included electricity, gas, and water in that — I would cover those costs myself. Her opinion: I’m trying to rip her off, because last year she also only charged me the utility costs.

I could rent the room out on Airbnb for a much higher price to tourists, but I would still give it to her under these favourable conditions, since she wants to open a shop and first needs to see how things go.

And now I don’t really know who’s in the wrong here. Maybe you can help me. Best regards :-)