I think Castle's parents gave each of their 3 children a lump sum when they sold their house to live in Castle's garage and be her childminder.
Castle ploughed it into her biz and renovated her kitchen (attraction marketing).
Sis needed hers for private cancer treatment. (Genuinely pleased she's had the all clear BTW although the Forever gift bags for nurses were a touch cringeworthy).
Bruv is spunking his share on his astronomical rent, plus stockpiling product for ccs. I don't think he would be posting so many photos and videos of his flat, the view, the gym, the pool, the bbq area, the window cleaners etc etc if he weren't so heavily invested.
The parents have been incredibly foolish with their (substantial) wealth - ignore the poor little me stories from Castle. There's money in that family. But it won't last long.
Bruv