Agree with some points that you made, disagree with others. She was hardly a parent and more of a sibling, and for me at least, all I could see her as was a long term babysitter. Even then, as a parent you should be more cognizant of the pictures you are sharing of your kid, she was quite literally sharing a video of a naked underage boy, this is disgusting regardless of being a sibling or a parent, and even more so if you have no familial ties. However, I do agree with your points about his unrequited love and how the lead has somewhat of an oedipus complex regarding a figure he sees as an older sister. It is evident that she cares for him, but it does not discount the fact that she is using him despite him being significantly younger.
I'm saying her actions at that moment are a "mom" thing.
It's not unusual for siblings that are drastically older than their younger sibling to sometimes have parent like moments.
It's considered "parent like" due to the fact that it's usually parents/parent-figures that do such actions cause in most cases, siblings usually have a smaller gap in age...
So sometimes, older siblings and uncles and aunts will have embarrassing moments they are fond of from the childhood and would act like a parent...
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And frankly, the reason we are now bothered by sharing baby and child naked pictures is more due to the modern landscape of knowing that people targeting kids sexually exists.
Before, this wasn't as much of a worry and having naked pictures of your really young kids wasn't considered a big deal
It's basically a really new and recent worry.....
And often times, parents/siblings often forget about that kinda stuff because we are often bad at thinking from other people's perspective... so since we can't sexualize the kids, we don't think about that stuff...
So if you think about it from the human history perspective, your the weirdo...
To her, it wasn't sexual, but instead an adorable happy memory. And while it's embarrassing, it's not really that big of a deal...