If the historic PCN was sent to her husband, has the registered keeper changed since then from hubby to mrs? Or did he open her mail? Just because "the car" has been clocked in the ULEZ before doesn't mean it was with the same RK, so that argument may be irrelevant.
The last V5c update was Feb 2021 and that well predates PCN Y338857187 referenced in the NOR, so the RK hasn't changed.
TfL reoffered the discount in the NoR. The discount window expires close of business on Sunday the 1st Sep. Our Administartor @cp8759 seems confident that if you submit an appeal to the Adjudicator within the discount windows then TFL will accept the discount rate if you lose the appeal. I'm aware that that happens with TFL PCNs, but usually when it involves a business and a parking contravention.
So if you have nothing further to lose by having a go at an appeal then I would do so. But it seems to me risky as to the discount. If they won't accept the discount rate if you lose the appeal then you would, or rather your sister, be in for £180.
Other than the NoR is some sort of a bad attempt at creating a one size fits all rejection I don't see much to put before the Adjudicator. It's a bit black & white. Were you in the ULEZ at the time alleged, yes you were. Was your car emmissions compliant, no it wasn't, so there is no exemption that applies. The Adjudicator has no discretion regarding mitigation so it's highly likely you'd lose. Probably the best you can hope for is that TFL "do not contest".
See what others have to say and PM @cp8759 and ask him to look at the current status of your cases.
Is it worth a shot with the Adjudicator? On balance I feel you might be best of paying it at the discount before Sunday.
