Yes, dress up together - Ambiguous path

I wasn't thinking straight. So I just did what she asked. I thought maybe this would encourage her to keep my secret. I was still shy. I turned my back to undress from my regular clothes. I took off my jeans, and Anna let out a noise of contempt when she saw the girly panties I was wearing under. Nowadays, when I get undressed, you can see the tan marks of a bra and thong on my butt. She likes that about my body.
When I tried taking off my panties, Anna stopped me.

"Not like that. When you take them off, you bend at the waist - not at the knee - so that you show off your cute little ass perfectly."

I did as she told, and I admit that I started feeling sexy. It was the kind of moves that I had already tested in front of my webcam, just to see how fuckable I looked when I moved. I kinda wanted to impress my big sister with my butt technique, so I made use of those long nights of practice when I was dancing, twerking, and stripping in front of the camera. I moved my hips in the sexiest way I could while slowly sliding off my panties.

She liked it a lot. After that, she started dressing me up with her favorite clothes from my secret wardrobe. Then she made me sit in front of the mirror and helped me with the make up. I admit that every time I did it myself, it was with a bit of exageration and I ended up like a teenager who could stop herself from putting layers and layers of make up.

Anna made me look so cute I started to find myself more attractive than her, my long hair helping a lot. The next day, she made me try her own clothes. And damn did I look hot in those! I was feeling sexy and relieved that Anna was taking this all so well. We started playing dress up every night, and not onyl me. She was playing as well, and therefore I could see her in underwear regularly. She was really hot so I had a hard time preventing my boner to poke out, especially while wearing a thong or thin lingerie under my skirt.

But that didn't stop me from feeling girly. At the end of the first week, she had let me do my make up on my own, and congratulated me on the result.

"You're really good at this, you know. You really look like the type of girl that any guy would wanna fuck. But tell me, what's your name ?"

I couldn't understand her question. Well, I did, but I couldn't believe it. Anna was trying to make me say something very specific. My boner started to move inside my panties. She asked again.

"Come on, I know a pretty girl like you must have a cute name. A name you use on your blog... Say it out loud for your big Anna."

"Selina," I muttered. She asked me to say it louder and with confidence. I looked up in the mirror, sincerely smiling at her and repeated "My name is Selina!" I was feeling good.

We kept playing dress up every night. Then, some evening when our parents were out, she dressed me up as usual, helped me with make up and my hair, and we spent the night casually, watching a movie down into the living room. I had never got at the first floor in girl clothes, so I was stressed out to be like this, in the open, . I kept looking by the window in case anyone would show up unexpected. Until my attention was dragged to the screen.
In the movie we were watching, there was a pretty hot sex scene. I felt warm, and I guess being in girl clothes made me super sensitive because I felt my dick hardening slowly. And the little thong I was wearing didn't hold it back at all.

Anna saw it and laughed a bit.

"You really are a horny little sissy, aren't you?" she asked while changing the channel to one with a parental code. She typed it in and revealed a porn movie where two girls were kissing. Anna stared at me.

"Come on, do it, don't be shy in front me, sis' !"
"Do what?", I asked in fear.
"Take off your panties and touch yourself for me."