Even artists that are living that fetch huge prices are not collecting the prices we think they are, especially in auction settings; there's more than one middleman. It's too bad when the artist's family isn't getting much of the profit. Though, even upon resale, by law, they are entitled to some (not much). Last I heard it was 5% in California -- of the retail. I would assume it's something like that in NY too, which would mean that his sisters stood to make a lot from the sale.
I just finished reading
an old article about the committee that his father championed before he died that authenticated the real ones from the fakes. This in fact helped to increase his prices and secure his legacy, etc. He died mid-2013, which explains why prices went even higher. Those are all definite authentic paintings, so I understand that it would be very difficult to authenticate them now.
In the case of Vincent Van Gogh - he only sold one painting in his life while he was alive, so being one of the highest selling artists of all time would probably very much surprise him. I would think it would piss him off. He'd ask why he had to suffer so much during his life, having to feel like a burden on his family, specifically feeling (probably at times) that Theo was cheering him on in vain. Yet we know that other times he resented Theo for not doing more for him. The whole situation of him not being recognized made many many things awful and complicated for him and probably exacerbated depression that was already there. So It's a shame. Many of the other dead artists we able to make their livings as artists when they were alive. Only few come to mind that had great difficulty. Sometimes I think of artists like Modigliani. While he sold work while he was alive, he suffered a lot of persecution.