I thought you handled it pretty well, Sho. Having made your statement, do NOT keep repeating it as it will only weaken it and come across as holier-than-thou.
She tried to trickle-truth you, as I'd expected.
The "I've never been happy" stuff is pure SCRIPT; it's probably stated 95+% of the time. I imagine that you have anniversary cards and hundreds of other shared family experiences with her that indicate otherwise. They say this to justify what the did; it's a "compartmentalizing" thing, morally.
You stood your ground; LET HER BE today. Remain upbeat, but not syrupy so.
I'm sorry she's continuing to lie to you; I know that stings.
