She HAS overstepped the mark.
If you read your OP you are all tied up thinking about your mother. She has got you focussing on the wrong people here!
It's your DS and your DH who should be concerning you, as they are the ones it affects. If your DH goes mad and doesn't want to see your mum, you couldn't blame him, could you?!
Difficult people that you have dysfunctional relationships with, esp in families, take up hugely disproportionate amounts of headspace. But it is not easy to untangle. I believe there is a long-running MN thread called something like 'But we took you to stately homes!' about just this. Worth a look?
In the meantime, please focus on your DS and DH. Your DS has been harmed by hearing this. You need to ensure no further harm by 'allowing' it to happen without being tackled. I think you have to let your DH know and ask him how he wants to proceed, and back him up. Make sure your DS is aware of this. Don't let this poisonous remark cause damage within your own family by 'permitting' it to happen unchallenged.
How your mum reacts to you tackling it is entirely her choice. Of course a reasonable person would be hugely apologetic and wish to make amends. I'm guessing she won't. But that's the bit she is in charge of. You're in charge of putting your DS and DH's feelings first and acting accordingly.