YOU SAID:
I peed in my sisters mouth last week
INTO JAPANESE
先週、姉の口でおしゃべりしました
BACK INTO ENGLISH
Last week I chatted with my sister's mouth
INTO JAPANESE
先週、姉の口とおしゃべりしました
BACK INTO ENGLISH
Last week I chatted with my sister's mouth
That didn't even make that much sense in English.