BTW, WolfishMusings, I have often thought of your sevara of familiariy, as well. Here is a limud zchus on those who don’t wear the full ragalia when davening, auch as the times when people are about to fall asleep at night and realize with a scream that OY VEY I DIDNT DAVEN MAARIV. How many can say they get fully dressed? (BTW, once heard from a yeshiva bochur that he saw his roommate davening in his pajamas and hat.)
Here is the limud zchus: We are supposed to dress like standing before a king. However, what if you are the son of the king, or the wife of the king. Does the king’s 5 yr old son dress in his suit every time he wants to speak to daddy? Same for his wife.
Klal yisroel are hashem’s children. Banim atem lashem elokeichem. Also we are compared to the betrothed of the king. So for a family member the halacha is different. 🙂