I don’t think anyone "deserve" to inherit. It’s just money you never worked to get and that you get because your parents had extra money.
So, morally, why would the brothers deserve more the money than the ex girlfriend ? Maybe they were still in good terms. Maybe she brought more joy in his life than his brothers. Maybe not but enough for her to get her share. Or just maybe he didn’t care enough about this and should be considered ok with the outcome.
You don’t deserve 1 more million on your bank account because you happened the brother of someone. Yes it may happen eventually but that’s just luck.
And I’m not taking side for anyone here. I can totally understand why the brothers would be upset and I would probably be if I were in their shoes. But at the same time, as someone who doesn’t count on his brother’s death to become rich, I’ll have a hard time being empathetic.
They should at least be upset about the carelessness of your brother rather than the ex girlfriend.
tl:dr; You are allowed to think that doing the right thing is not to give the money back because it was never meant to be theirs.
In this case, as you say, the guy didn't put much care into the issue, so we don't have to worry too much about if our decisions are what he would have chosen. What he chose was to barely consider the matter and mostly leave it up to others to figure out.
The fact he did remove the 2nd gf is interesting but probably has 2 arguments that cancel out. A, he removed the 2nd ex from insurance as soon as she was an ex, so in general he doesn't intent for exes to still be beneficiaries. B, he removed the 2nd and only didn't remove the 1st, so keeping the 1st on was intentional.
I do not think the 1st was intentional, but not taking care of this stuff in general was intentional, so he gets what he gets.
The brothers, well it's unfortunate that unfortunately they had a careless brother. If the court gives it to the ex, no great travesty has happened. The state is only deciding because the guy largely didn't bother deciding for himself.
But the lower the stakes, the easier it is to show integrity. Doing the right thing at no cost is easy.
Integrity isn’t just about doing the right thing when no-one is looking.
It’s doing right thing, even when no-one is looking, no matter what it costs you.