If I’m staying at someone’s house, I will bring lounge clothes to wear first thing in the morning if I don’t want to get fully dressed before coffee. Joggers and a T-shirt or sweat shirt. Comfy, but something I could wear out of the house without scandalizing anyone. I also sleep in eberjay PJs and I wouldn’t wear them out of a bedroom except at my own house. I also wouldn’t even be around my own kids without a bra on, maybe your sister assumed you weren’t wearing one if you were still in your sleeping attire. Seems like your sister is a little more formal, so get dressed next time.