I travelled with my Ladyboy gf to a village in Northern Thailand where she was born and raised to meet her mother and sister. We stayed with them at her mother's house. It was a small house with no privacy so we couldn't have sex. The second day, my gf was horny and needed to cum so she took my hand and led me to the bathroom. She put me on my knees in front of the toilet and pulled out her cock. I went right to work sucking her cock deep and she cum in a few minutes. She shot a big load, I swallowed most of it but some leaked out of my mouth and ran down my chin. I was massaging her big balls with one hand and milking the last few drops of cum out with the other hand when the door opened and her sister was standing there. She smiled, walked around us, lifted her skirt, pulled down her panties and sat on the toilet to pee. I licked the last drop of cum from my gf's cock and stood up to wash my hands and face. They were having a casual conversation in Thai like nothing happened. Since that day her sister always gives me a sweet knowing smile when she sees me. She knows I am her sister's bitch.