YOU SAID:
I am masturbating to my sister
INTO JAPANESE
妹に自慰行為をしている
BACK INTO ENGLISH
Masturbating to your sister
INTO JAPANESE
お姉さんにオナニーする
BACK INTO ENGLISH
Masturbate to your sister
INTO JAPANESE
お姉さんにオナニーする
BACK INTO ENGLISH
Masturbate to your sister
That didn't even make that much sense in English.