http://www.typologycentral.com/forum...ehaviours.html on the problems with justifying someone else's poor behaviour, so I won't repeat it too far in this thread. Forgiving them for it is another matter: but when someone is on the recieving end of this kind of devaluing treatment, their first responsibility is to take steps to protect themselves. Feeling sorry for someone who is treating you poorly is not the way to bolster your own self esteem, or to push them into giving you better treatment, regardless of whether they are actually intending to hurt you or not. This is the sort of reasoning that can lead to someone turning themselves into a victim, and I really wouldn't recommend it.
Superkumquat would know better than anyone else how much her mother makes a habit of this sort of thing, and having underlined the issues which her mother may be bringing to the table in this instance, I would hope that she is able to decide for herself how much of a problem it is at other times.