Theory: The better you know someone, the fewer manners are required to engage with them.
Here “manners” are defined as the rules and customs your parents taught you so that you wouldn’t embarrass them in front of their parents and friends (Your parents were taught manners by your grandparents for the exact same reason).
Corollary: Presenting the above theory to family, no matter how well you know them, will not be accepted as an excuse for lack of manners.
Praxis: When I sneeze in public and a stranger says “Bless you,” I say “Thank you.” When I sneeze in the company of friends and someone says “Bless you,” I say nothing. My soul is not leaving my body, and I don’t have the plague. There.
Another: a haircut is not worth commenting on unless it fundamentally changes a person’s appearance. If someone gets a haircut merely to trim back or shorten their existing hair style, nothing needs to be said. This kind of haircut is upkeep, basically an act of hygiene. We don’t routinely compliment people for taking a shower or clipping their toenails.