if this sentence correct?

The highlighted part is correct. It refers back to the first sentence. It seems he is totally disappointed with those selfish adults (who could afford to, but would not help their parents). Therefore, he swears that if he could earn more money, he would buy his mum a dream home.

The key structure is: " if I could, I would", and is used correctly in the highlighted portion of the sentence.

他的文法理解正確，當以假設性/預計未知的情況，在if用past tense是正確的，只是意思上用could即意思是can earn可以賺取更多錢時，不過可能這是中文習慣語，英文人的賺錢是得到才算的，所以要買到樓即是要已經賺到所要的錢，而不是可以賺到這筆錢，所以比較地道英語的寫/講法是：
I would buy you a dream house if I had earned more/enough money.
這句看來應該順耳些。

Could

(用於假設語氣, 表示與事實相反的設想)能, 可以
I wish I could go to France. 但願我能去法國。
I would go if I could. 能去我就去。
(yahoo dictionary)

I would buy you your dream house if I could earn more money.

The above sentence sounds alright. He is telling her mother (in a negative tone) that he cannot afford to entertain her unreasonable request.

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希望可以幫到你!

totally disappointed wif those selfish adult~i'm really hurt ~
I am really hurt and totally disappointed with those selfish adults .

Hey mum! i swear ~i'd buy u ur dream house if i could earn more money ''
Hey Mum!
I swear that I would buy you a dream house if I could earn more money.