My older sister[19F] makes me expose myself[18M] completely every time I meet her. Update:1

So this started at my sister's birthday party, being from a rural background in the Indian subcontinent I was never given much privacy. So I am mostly naked in the household while everyone is clothed as everyone is older than me and me being only 18, being naked is okay. So this one time during my sister's birthday [19F] I was [18M] she had just gotten a camera and wanted to click pics. I was already naked and she was receiving gifts from my family, like mom, aunt and grandma. So she asked me for a gift and I not having any said I would do anything you ask me to do today. Everyone laughed at it and so did she and then she asked me to bend over and spread my ass in front of her. I initially laughed not because I was ashamed but because it was weird but then my mom and aunt thought it was funny and encouraged me. My mom saying it's your sisters birthday so do it and you look cute anyways like that and my aunt saying you are already naked so what are you ashamed of. I also not having a reason to say no agreed. My sister made me spread my legs while my mom bent me over and aunt took my hands upto my ass to spread it. Every one thought it was funny and my mom spanked me lightly saying good boy but my sister took a picture of it with her camera. I initially hesitated but after aunt pointed out that most of my pictures today was of me naked I backed off.

Fast forward to some time later she asked me to do the pose again but I refused so she kept pestering me saying you are already naked just do it, its just harmless fun. Annoyed I did it again. This became a routine so every time I meet my sister I have to strip naked first and then bend over and spread my ass, and I could only get up after she felt it was good enough by spanking me. This became common tradition even in front of my mom who thought it was a childish thing between a brother and a sister.