YOU SAID:

i thought that maybe he was better off their sister instead if their mother

INTO JAPANESE

母親がそうするなら、妹のほうが良かったのかもしれないと私は思いました。

BACK INTO ENGLISH

I thought that if my mother did, my sister might have been better off.

INTO JAPANESE

母がそうしてくれたら、妹はもっと良くなったかもしれないと思いました。

BACK INTO ENGLISH

I thought my sister might have been better off if my mother had done so.

INTO JAPANESE

母がそうしてくれたら、妹はもっと良くなったかもしれないと思いました。

BACK INTO ENGLISH

I thought my sister might have been better off if my mother had done so.