My sister and I don't know what's happening between us.

My (17M) sister (16F) and I got snowed in at our cabin. We're alone for the first time, and we're enjoying each other's company. Our parents are away for work, and we're trying to make the most of it. We have been doing some typical snow activities, and I noticed my sister was looking at me differently. It's like she sees me as a guy now, which is a little weird. I've been checking her out too. She's grown up a lot. Our interactions have become more playful and flirty, like we're trying to impress each other or something. I feel confused, unsure how to react. I know I'm attracted to her, and I can't help but steal glances, but I don't want to ruin our relationship. I'm just trying to figure things out, but I don't know what's happening between us.