Herge's nephew and godson, Georges Remi jr, has written a book "Un Oncle nommé Hergé" (French for "An Uncle called Hergé") in which he describes his relationship with his Uncle Georges and also dwells on Herge's relationship with his brother Paul.
According to the reviews, it also focuses on the conflict between Herge's relatives and Nick Rodwell, the head of Moulinsart, who is criticised for the way he exploits Tintin while failing to maintain the spirit of the series.
I've read the opening pages of the book. I can't say that the Remi family strikes me as particularly close. It starts for example with an incident in 1979: Georges jr. was himself an artist and had invited his uncle to the preview of an art exhibition of his work. Herge has sent a letter, typed out on a typewriter - as if by a secretary - declining the invitation due to an argument that they had had earlier and for which "you [the nephew] had not yet had the decency to apologize for." Georges jr. describes himself as being furious with this snub - though he does not mention the details of the argument itself.
This book, along with Benoit Peeters' "Hergé, Son of Tintin", indicates that Paul was very frustrated over the five years he spent in a POW camp during the war, especially given the fact that his brother, rather than help in the resistance, spent the same period working for "Le Soir", a newspaper which collaborated with the Germans.
What is more, while Paul's military career made little progress, his brother's went from strength to strength and these factors led Paul to drink. It seems that Herge was even concerned by Paul's parenting and suggested adopting his children. The children themselves did not like this idea, due in part to the fact that just visiting Uncle Georges meant being very quiet and careful about the house and Georges jr. would get so bored that he would ask to be allowed to leave early rather than stay any longer. Denise, the daughter, even resented the way in which, while she was still at school, Herge would answer her letters with the same wavy handwriting and spelling mistakes.
Georges jr is himself an artist, much of which is centred on the sea. His work is very nice, especially the way in which he catches the waves splashing about the ocean. Here's a gallery of some of his paintings on his website