退一萬步講若我同老公離婚了=>However, you can also 爭財產 for your husband's flat, right?
You are not live in your husband's flat now, however, you are living in your 99's flat. How about do you change the living place with your husband's brother? So that you will feel fair.
Do you give living expenses for your 99 now? (and before ?)
If not, then just think the 1/3 flat is the living expenses for your 99, and forget it.
Work hard (you 2) and then you can buy your flat with your husband.