Is she under stress? Under those circumstances, the main time I would get a little "suck it up, buttercup" esque with those close to me is if I'm at the end of my string already. Detaching becomes a form of self-preservation or else you'll just break completely. Even so, I think I would probably be smitten with guilt after the fact, especially since you're a reasonable person and not someone constantly bringing up unreasonable frustrations or complaints. Could be her shadow self?
The second possibility I thought of is that both you and the INTJ are Ts. Your mum is an F. Therefore, she may end up reading the situation as being more emotionally charged than it is (and consequently she wants to stay away from "taking sides").
Do you think that your mum agrees with your aunt's complaints (eg text messages), even if she may not agree with the way the message was delivered and therefore doesn't want to discount them, even though she would like to be supportive of you?
Just throwing out some possibilities...
Do you know your mum's e type?