It's taken me almost two years to establish that my mother's an ISTJ since I thought she was ISFJ (gender roles threw my investigation off). Typing my friend as ISTJ tipped me off to my mother's true type. Every ISTJ I'd met before has been abrasive and domineering. My friend and my mother are very reserved and gentle, they put on an image of being contained so it's difficult to recognise how wounded they are since they carry their burdens patiently. The title duty-fulfiller fits, they're bound to them and sacrifice their positions for others and, unlike ISFJs, you won't get passive-aggressive behaviour as a result (which is a virtue and a vice). They can be surprisingly passive with accepting these duties although when you question them, they emphasise obligation and commitment. In short, the flipped INFP - the silent martyrs. The word "duty" (encompassing terms and synonyms) doesn't exist in my dictionary.
My friend confined in me that it was comforting to have someone to vent to however I noticed that she never complained about the duties that bothered her. I put the pieces together and realised she was sacrificing her career choice for her family but then I realised that she didn't seem to even know how unreasonable their expectations were because she was so wrapped up in responsibility and reliability. This is similar to my mother who's been caring for my ungrateful grandmother, when I tell my mother to confine in me, she'll only complain about trivial aspects. It took five years for her to discuss the issues she has with her mother even though she said she appreciated having an ear, she was still very cagey.
... I wonder if their guarded nature, caused by weakness in determining the trustworthiness of others, is the effect of inferior Ne? I notice INTJs are quick to grasp the virtuous nature of people and I think it's because Se is tangible (that inner ISFP).
I'll have more thoughts to add later.