No, I don't think it is sinful.
I don't think that the money that JK Rowling has earned through her labour and genius and through the pleasure that she has given to millions of children and adults is sinful at all. On the contrary, I think it is magnificent.
What I think is sinful are people who want to restrict the freedom of other people, who want to remove what is theirs, who cheat and steal from those people with expenses for bath plugs and duck houses and moats etc from the public purse, or who operate from "charities" and draw large expenses from them. What I believe is sinful is people who lock other people up for thought crimes, send them to gulags for daring to oppose their worker's paradise or wanting to read the paper of resistance - the Daily Mail.
What I think is sinful is those who set themselves up above others and tell them what they should do. What I think is sinful is a rich elite of royalty and privileged fat cats who deceive the public by trying to frighten them out of their wits that the "planet" will be destroyed and tell them to live "sustainably" while they live in mansions with castles and acres of land and have never had to worry how they will be able to afford their next fuel bill, which has been artificially and deliberately inflated by the elite in order to keep the hardworking decent people down.
What I think is sinful is an elite that starts wars to rob the resources of ordinary people in order to enrich themselves and impoverish and starve ordinary people. What I think is sinful is those who fund Jihadis and Al Qaeda and let them destroy a civilised country and decapitate people and eat the lungs and hearts of captured soldiers in order to enrich a privileged elite who want to rob the resources of that country and its people.
What is sinful is anyone who wants to restrict the liberties and right to growth and progress of the people. Capitalism and free enterprise are not sinful because they re a route to wealth and wealth is not sinful because it buys freedom.
If the "carbon footprint cabal", the "sustainable swindlers" allowed people in poor countries real free enterprise and capitalism, then there would be fewer poor and starving people on this planet, just as there are now fewer poor people in the UK or South Korea or in any other country that practices capitalism. But that is the exact opposite of the wishes of the "carbon conmen", the "sustaninable swindlers", the "ethical" elite, with their "charities" and their "expenses" and their wish to deprive the people of the world of growth and wealth.
It is not sinful to want to work and study hard and want to live well and earn "loads of money" and enjoy some of teh luxuries and freedoms in life that are enjoyed by the elite who preach "sustainability" for everyone else apart from themselves.
If you think it is sinful that you have money and others are starving, then give your money away, but don't tell others that they are sinful because they struggled and sweated to earn the little that they have.