Why did my ex boyfriends sister follow my instagram?

Anonymous
Okay so a few days ago my ex boyfriends sister followed me on instagram. Which is weird to me since my ex and I didn't end on good terms. About a week ago I posted something on my instagram story and a few hours later I looked at who seen my story and I noticed my ex boyfriends sister looked at it and she wasn't following me so it was weird. Then about 2 minutes after seeing that, she followed me and I followed her back because I don't have a problem with her.

She doesn't like any of my posts, but she looks at ALL of my instagram stories and she's actively on instagram as well. I'll see she likes other people's posts, but not mine. My friend who's known her forever requested to follow her twice and my ex boyfriends sister declined and she also doesn't follow another one of my friends that she used to be best friends with. Can somebody help me? I'm kinda curious because it's weird to me.
Why did my ex boyfriends sister follow my instagram?
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