My son is so pure than his sister always makes him cry
長男 - oldest son, son
妹 - younger sister
優しい - kind
この場合は weak (弱い）, pure （ピュア), gentle （ジェントル）も言える
いつも - always
泣かされる - made to cry
My son is so pure that his sister always makes him cry
Our son is so tender-hearted, his little 4 year-old sister often brings him to tears.
この場合、次のような言い方ができますよ。
ーOur son is so tender-hearted, his little 4 year-old sister often brings him to tears.
「うちの息子はすごく優しくて、良く4歳の妹に泣かされてる」
to bring one to tears で「人を泣かせる」
ご参考まで！
イギリス 
カナダ 
