YOU SAID:
my sister is brushing her teeth next to me
INTO JAPANESE
私の妹は私の隣で歯を磨いています
BACK INTO ENGLISH
My sister is brushing her teeth next to me
INTO JAPANESE
姉が隣で歯磨きをしている
BACK INTO ENGLISH
My sister is brushing her teeth next door
INTO JAPANESE
姉が隣で歯を磨いている
BACK INTO ENGLISH
My sister is brushing her teeth next door
You've done this before, haven't you.