The OP's opening post and thread title suggested that she was taking offense because the remark was directed at her mother and not because it was sexist. I think a lot of the pre-update comments reflect that understanding. The comment wasn't directed at her mother, it was directed at women. If her only concern is her mother, then I believe it is an over-reaction.
If she is particularly offended by sexist comments as indicated in her update, then that is something else entirely. I'm puzzled as to why she emphasized that the comment regarded her mother, if that is the case. That I think should could comment on.
From the OP: What I want to write is in no way ehell approved, but goes along the lines of "That's MY MOTHER you're talking about!"
I would view any comment like that as petty and silly. It implies that the same comment about any other woman would be okay by you. I would find that inappropriate. Responding to the sexist nature of the crack would not be out of place, IMO. And not necessarily rude depending on wording.
And yes I'd feel the same if it were a nasty remark about any other group. Inappropriate to base your objection on it being your mom when it obviously wasn't referencing her personally. Appropriate if the objection is based on the bigotry of the statement.