It all depends on your definition of success. I do not mean the money v happiness question but, who is the more successful? The one with all the money or the one who reaches the highest point in a poorly paid profession?
I am the eldest, my salary exceeded my younger sister's, but I was a small cog in a big wheel and she was director of the heritage body that employed her. My youngest sister also had a successful career, until she decided to give it up and do something entirely different
DC present a similar problem. DS, the eldest, without doubt is doing better in all measures, money, seniority and professional acclaim, but DD, was doing well but then she consciously decided to opt out of the rat race and do her own thing. She has a job she enjoys and is one of the best paid in a job where pay is based on skill but it's main advantage to her is that it makes no demands on her outside working hours and she has quite a lot of control over them so that she can pursue her other interests. She could go for promotion and would probably get it, but she has chosen not to. She is single and childless by choice so balancing work and family do not come into it.
Seeing the demands placed on DS by his successful career, which he loves, great success comes at a price.