i really like seeing the mother/daughter sharings and I do remember wearing some of my mom's clothes, particularly when I was in my late teenage years and needed something grownup for a particular occasion. But i don't think I'd enjoy wearing my mother-in-law's clothing, even if she were the grand poo-bah of Ruritania. There's something that just feels creepy to me about that. Accessories and tiaras, certainly, and maybe for a state occasion a re-cut evening gown. But not everyday dresses and the like. I would feel judged and compared with every public appearance.
That said, these folks spend so much money on clothing that maybe they regard their closets as full of interchangeable uniforms. And I'm sure that any garment that gets passed around is in exquisite condition with no visible wear of any kind. Still, for me, there's a yuck factor.