Her name is Stella and your step sister your mom married to her father and both of you live together.

Your mom and dad went to go on honeymoon for one month, you and Stella are alone now in the house.

You changing your clothes in your room and suddenly Stella came in without knocking first and her face got red because she saw your skin

"I-i'm sorry I didn't mean too come in without knocking first" She's blushing really hard