Nan and the maids are mean to Marnie because she's living there during the post WWII occupation of Japan. Assuming the story takes place in the modern day, Marnie would have been a girl around WWII. During the war an obviously western family wouldn't have been allowed to live there. However, after the war the Japanese would have had no choice but to allow American businessmen. Nan and the maids don't dare take out their resentment of the "foreign invaders" on the parents but a young girl that's all alone is a different matter.
Oh wow. That makes a crazy amount of sense. Probably not entirely intentional given (I assume) it was the same in the book which wouldn't be set at the same time. Though for the adaption the dates line up near perfectly if we assume Hisako is in her early to mid-sixties (and it's set near the time it was published).
Marnie and Anna's story absolutely plays out like a love story. And yet Marnie is Anna's grandmother. But look at their interactions: Anna is the one blushing and staring in awe. Marnie is delighted to be with Anna and says she loves her but doesn't seem flustered or infatuated in the same way. Because if you take her by her words when they first meet, she's watched over Anna and knows just who she is. In fact, her vocal cadence sometimes seems quite grandmotherly, ditto for the way she comforts Anna or teaches her a skill.
Anna, on the other hand, is framed as having a romantic friendship for Marnie at the very least. So, learning what we learn, what's the point of that? Anna isn't just struggling with her feelings of abandonment, but with her identity. This could be her sexuality, but it's also more explicitly her ethnicity. She hates having anyone look at her at first, freaks out when people notice her eyes, and says she thinks she's ugly and an outsider. She doesn't like wearing traditional Japanese clothes or attending festivals because it highlights her otherness. Being infatuated with Marnie and the mansion, feeling compelled by them as if in a forbidden romance, could represent feeling torn between her unknown past/Western background and the Japanese family that she still doesn't see as 'hers' yet. This also explains why when she thinks about Marnie, she forgets the relatives she's living with and vice versa. She doesn't yet see both of these things as a part of her identity, until she learns that the girl she's drawn to is part of her history and her background - and Anna becomes able to fall in love with herself.
When Marnie apologizes to Anna for leaving her in the silo, her vague wording of said apology makes it so could also be read as her elderly self apologizing to Anna for dying when she was a small child, thus abandoning her, albeit unintentionally and involuntarily.
Marnie hints at not being in the same timeline as Anna when she explains why she disappears from time to time. When Anna confronts her at the end for leaving her behind at the silo, Marnie says something to the effect of "at that time, you simply weren't there", referring to the fact that Anna wasn't born when the actual event occurred, as with anything Marnie detailed in her diary.
