It's rude to flaunt wealth.
John notices that his niece and nephew are wearing old, worn out clothes.
John wants to get some new clothes for the kids.
John asks his sister if it would be alright to take the kids out, because he wants to spend time with them and buy them a few gifts.
John goes to his sister and brother in law to let them know that he has plenty of money and wants to help the kids to dress better.
His sister and brother in law are not offended by his desire to spend time with the kids and are happy to see them get some gifts.
His sister and brother in law are offended that he is insinuating that the kids are not being taken care of and decline to accept his help.