Sister dropping her bra (pre story)

My name is Marc. I’m 18 years old and live at home with my parents and my two older sisters. My sisters are Margot and Sofia and they’re 23 and 21. So I’m the youngest in my family.

“Marc?” Sofia said, when she came down to my room.
She’s not afraid of interrupting me to ask if I want to do something, but I usually also just have my door open so it’s fine.
“What” I replied.
She stepped into my room with a lotion in her hand. I could see she had been showering. She had a towel on her head and she was wearing nothing but her underwear. Her bra was a skin-toned and had matching tight fitted knickers. She exercised a lot and was in an unarguably good shape. I knew she looked good. Even my friends told me I had a hot sister.
“You think you could help me with my back?” she asked, while unscrewing the lid of the lotion. I paused the YouTube video I was watching and stood up without answering. She walked to her room and I followed. She opened up her bra and held onto it with her hands cupping her breast. I put some lotion on my hands and started rubbing it out on her skin. I got it down to the bottom of back and touched the top of her nickers. I dragged my hands up along the sides of her and felt the curvature of her body. I have to admit that what I felt was nice. I dragged my hands down along her sides to “ensure” that I spread the lotion well. As I did she jumped a little. I knew she was very ticklish.
“Did that tickle?” I teased her.
“Stop!” She responded with a voice that tried to be angry.
I knew she wasn’t really mad, but just hated to be tickled.