Hey @s3_gd7y-bod0l, thanks for writing!
You probably subscribed to the Family plan using your credit card as the payment method, but ended up using your brother account to do it.
About your sister, sometimes the information once given to Spotify (regarding the users’ addresses) don’t match the ones they have. The data might be incorrect, incomplete or outdated, hence the removal of some users.
In this case I suggest you get in touch with the Spotify Cares staff - They'll be happy to help you further with this.
There are a few ways to contact them. One is filling out the online contact form.
Another great way is to reach Spotify via Twitter - just send them a Direct Message @SpotifyCares.
You can also use the Facebook support. Shoot them a message here: Spotify Cares.
When you message them, inform all the e-mail addresses on the plan.
Let me know if this helps!
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Note: I'm not a Spotify employee.