Sigh ... your parents are treating your brother too well from the beginning. I don't know how much your brother is paying for the mortage every month but i don't think the money (10k / 20k / 30k ??) he's paying is so much to be considered as mortage re-payment. If he did not pay 首期+舊公屋, he is not the owner. I think your parents should think that they are just renting part of the apartment to your brother and his wife. The so-called mortage payment is just the rent + baby-sitting fee. Your brother is an adult and a married person. He is just paying for his own part of living (himself + his wife + his bb x2 ).