Let's do something similar with a simpler sentence: "She looks great - unquestionably so." "So" refers back to great: "she looks great - unquestionably great". In your example, "so" refers back to "her 88 year old self": "She looks her 88-year-old self – magnificently, imperiously, but relatably her 88-year-old self".

'So' is replacing the entire beginning phrase - she looks her 88-year-ols self - so that the writer doesn't have to repeat it. It means that she looks her 88-year-old self in a manner that is magnificent, imperious and relatable.

as a translator I would suggest that even though she looked magnificent she looked her age of 88.

It means "in a way that is relatable". In other words, although the fancy bags and designer clothes give her a magnificent and imperious air, they don't hide the fact that she's an elderly lady. She's relatable because physically she looks like any other elderly woman, so when you look at her face it reminds you of an elderly woman you know or knew, especially one who pushes or pushed the boundaries when dressing themselves. The author gives the example of an aunt, but it could equally be a friend, an acquaintance, a grandmother or the crazy lady who lives across town. It's not common in everyday speech, but it does exist and it's good to understand it.

It would make sense if "magnificent" and "imperious" were in their adjective forms. i.e. magnificent, but in a way that's relatable; imperious, but in a way that's relatable