I probably buy about 75-80% second hand although less over the last 18 months because I prefer to buy it from real shops (although this may have to change). I think I'd struggle to get the figure much higher. This is partly because there are certain items, that I very rarely buy second hand, e.g. shoes (unless clearly unworn) or T shirts (because they tend to be in poor condition), and partly because it's a personality trait that if I forbid myself something completely, I become obsessed with it, and partly because if I want something specific, it can take forever to find it second hand.
It's time consuming - this is fine for me because I treat it as a hobby and really enjoy the hunt. But if I still had care responsibilities it would be a struggle to find the time.
I think one thing many of us are guilty of (although of course I don't know your brother or SIL) is bragging about the gems and the bargains we find while keeping quiet about the mistakes - I make lots of them because it's always a one-off purchase and, if you don't snap it up now, it probably won't be there next time. Just as an example, I bought some trousers this weekend that are two sizes too big. I've just washed them on 95 degrees to try and shrink them but I'll probably end up redonating them. Not a biggie in itself but It adds up.
I'm also suspicious of how corporate the whole second hand business is becoming. The last time I was in the Manchester northern quarter, I was struck by the number of businesses that were clearly chains, some of them selling 'replica' (i.e. fake) vintage. And much as I like a weigh n' pay sale, I do wonder who exactly these companies are that make money out of it.
Sorry if some of that sounds a bit negative. This is a hobby that gives me immense pleasure and that enables me to buy some fabulous clothes I could never otherwise afford. But the business is changing and I just have a sense it might not be quite as ethical or sustainable as it makes out. Good luck!