translation - Figurative language ひどく下品な上に

Reading 幽霊塔 by 江戸川乱歩, I have come across a rather long sentence:

第一、栄子は女学校は辛うじて卒業したけれど、私の眼からは、低能児同様の無教養に見えるし、素姓の知れぬ乳母の連れ子だけあって、ひどく下品な上に、意地悪なことは人一倍だ。

Here is my interpretation:

First of all, even though Eiko barely managed to pass high school, from my perspective she seemed like an unrefined feeble minded child who was just the daughter of some ill-bred divorced wet nurse, and to the upper that is extremely vulgar, was malicious more so than others.

I'm not sure how 上 is functioning in the phrase ひどく下品な上に. Is it correct to interpret it as meaning "upper"? I would like to read some other perspectives on this phrase and how it fits into the sentence.

Answer

上に in this sentence means " in addition". Your sentence is translated as " In addition that She is extremely vulgar, is malicious more so than others."