There is a saying a fool and his money are soon parted, if elder brother lost his job or became ill a lot of the trappings of wealth could disappear. I wonder how much is on credit now?.
All 9 of the in laws in my husbands large family felt left out, there was always 2 or 3 of us who managed to gather in a different room and chat amongst ourselves, eventually someone, usually my husband, use to come looking for us, over the years [almost 40] he came to realize that his family were not the ones he wanted to be with, and even the sister he was very close to has become someone he hardly contacts.
Do keep in touch with his parents, you say they are friendly with you, they won't say anything but they may not like the elder brothers' wife and her show off ways. They are a different generation they may think designer clothes with their inflated price tags a waste of money, may believe that as long as the car goes it does't matter what make it is. They may worry about what would happen if the money ceased, they may have known people like your brothers in law in the past and have seen them loose everything.
Stay as you are, do still go to the family get togethers no matter how boring, just before one is due surf the net to find what styles or colours are in or the latest gossip, at least you will know what they are on about.
Try to keep friendly with younger brother his relationship may not last, and you may find something in common with his partner, ask just them round for a meal or go out for one, Chinese or Indian, not a posh resaurant. I am sure you will find something to talk about, forget her money, treat her like any other girl you know, you never know she might not really like the other in law.
if you need a new outfit go to Primark etc for the in colour