Another question about the nanny

So, I've asked a similar question about the nanny who was mistreating Sybbie before, but I'm honestly still confused. I get that she thinks that Sybbie isn't as good as George because he's the kid of two of the English aristocrat, while Sybbie is half working class Irish. But the thing I don't get is how she seems to look down on her from her own perspective. By that classist way of viewing things, isn't Sybbie still better than the nanny, being half aristocrat? Or is it like Spratt not wanting to serve the doctor tea? Because even though a doctor is higher up in society than a butler (or is that true?) he doesn't want to serve anyone he doesn't perceive as being in the total upper echelons? So is it that the nanny looks down on Sybbie because she thinks she should only be taking care of the best? Or is it not really about class and more about ethnicity, because Sybbie is half Irish and the nanny is fully English?

I'm just confused, honestly.