Above all, I really like that your parents are having this conversation openly now!
I think it’s a difficult one (putting aside scenarios involving disabilities). It’s all fine and good to have guiding principles about different careers paying differently in spite of equal efforts/skills/societal importance etc, and about life experiences until that point, but it’s not always that straightforward. I suspect my sibling regards it as largely luck that my salary is 2x hers, and certainly I don’t dispute that she works hard and some of the disparity is luck, but it’s also true that I am 10 years older, have a MSc whereas she has a level 3 qualification, and routinely work well over my hours! At the minute, I’m a single parent with a heavy mortgage and she has a partner and rents, so right now I’m very stretched and she has a much easier day to day quality of life, so tbh an unequal split in her favour right now would be tricky to stomach. 10 years down the line, with a much bigger dent in my mortgage, i think it would feel very different. (In practice, I don’t think there’ll be much/any of an inheritance for us anyway, which makes life simpler I guess...)
Adult DC make life choices, including which job to do, based on all sorts of things. I love my job but if money was no object, it isn’t exactly the one I’d choose. In relative terms that’s a lucky position to be in, but again comparing to my sister, she chose exactly the job she wanted to do. I wonder if your DB is coming a bit from this angle, that it wasn’t as straightforward as ‘his vocation was more lucrative than DSis’s vocation’ and an unequal inheritance sort of upsets his own balance there, in that he feels he compromised for pragmatic reasons and DSis has been able to truly ‘have it all’? I don’t think that’s exactly fair (I wouldn’t feel this way about my own DSis) but it’s one perspective.
I think also, looking at my wider family and others I know, that non-inheritance stuff is also often unfairly divided (some siblings benefiting from more help with childcare or living with family while saving up or whatever - sometimes because of favouritism, sometimes because the timing or geography worked out that way). It’s very difficult to make everything the same for all siblings, I guess the temptation can be to try to compensate for other things via an unequal inheritance but I think as soon as you introduce that element of comparison and judgment about ‘the right amount’, it becomes really fraught. For this reason I think I’d choose to split it equally between my DC.
Finally (too much to say 
) I think that usually when families become resentful about inheritance, they’re mostly acting out resentments that were already there. If everyone feels valued and loved and genuinely likes their siblings, I can’t imagine how this sort of bitterness can fester (or how they can let money take centre stage in the context of parental death!). Not generally judging the adult DC in that scenario, I think it’s often symptomatic of wider dynamics they didn’t create, but I often think this gets overlooked in theoretical discussions about what makes for a “fair” inheritance.