I haven't read all the posts so my comments may be redundant but this is how this Fi user would react to such a situation (read: no offence intended to ISTPs or Fe-users in general):
Regardless of the ISTPs intentions, I must say there a few things I find more infuriating than conversations such as that (and I do have them regularly with my ISFJ mother). Its just full of my least favourite conversational elements: circular arguments, character assassination, self-righteousness, judgementalism, guilt trips, presumptiveness, playing the martyr and strawman arguments - these offend my Te as much as my Fi. Orobas, I really identify with your thought process and I doubt I would have remained as tactful as you did. It bothers me so much to see people punished over and over for mistakes (many of them probably unintentional) they are attempting to change and overcome them.
In fact, it seems to have rather irritated me just by reading it...
I think I would have actually preferred to hear the ISTP say outright (and here, I will channel my ESTJ sister): "I know its terrible to say but that woman and her children really disgust and irritate me. Yes, they've been through all these terrible troubles but can't she get her sh*t together and feed, clothe and take care of her kids properly. I can't wait till they leave." At least then she's acknowledging that she's being bitchy and unreasonable rather than trying to justify the harsh criticism through BS arguments.