If you staying with your in-laws, how do define improper attire? Does not wearing a bra considered as improper?

Improper attire according to whose definition, your MIL or yours? Males walking around topless, in boxers, with only towels wrapped at the bottom after a bath, teenage boys running naked after a bath? Or females in translucent clothing, no bras, walk around in towels/ nightwear/ bath robes, dressed in oversized T-shirt and no shorts etc. Different people have level of tolerance and comfort levels. Check with the person directly to ease your mind.