My ex’s new girlfriend’s sister requested to follow me on instagram, why could this be?

I just got a request on instagram off my ex’s new girlfriends sister. I originally accepted it to see if she would message me or something but I quickly decided to block her as I don’t want any potential drama. Is it likely that my ex’s new girlfriend got her sister to follow me and why do you think she would do that?
Also I think my ex has only just started dating her but they started talking as soon as we broke up and are now dating just over 4 months later since our breakup.

My ex’s new girlfriend’s sister requested to follow me on instagram, why could this be?
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