If your dad can prove your brother never paid any money towards the down payment or mortgage payment and your brother never shared the rental income, that is good enough proof.
Marital property to be split in a divorce is property acquired during the marriage, not before the marriage. This is HK, not America.
Go to District Board member to ask for free legal advice. I am not a lawyer but acc to what I hear fr other people who got divorced, this does not work the way your lawyer told u.
Of course she can sue, but it does not mean she can WIN.
Of course bank records is the best proof. Even if your dad did not keep the records,if your dad paid cash one time with no mortage and your bro was just a student then, that can also prove yr bro never paid anything even if u do not have bank records. If there is a mortgage, was it repaid before your bro started working?
Talk with the woman (wife of your bro), tell her your bro never paid any money and beg her to leave your father alone as he needs the money for retirement. talk nicely. don't scold her.