Could it be phishing trying to get you to log in and give your account information to some bad person? Have you checked your System Settings and does it say you are a member of a family?
Scams (e-mail, text messages, and phone calls) are getting very good at closely imitating true Apple communications. Always be cautious. These support articles have some guidelines:
About identifying legitimate emails from the App Store and iTunes Store --> Identify legitimate emails from the App Store or iTunes Store - Apple Support
Recognize and avoid phishing messages, phony support calls, and other scams - Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support
Avoid scams when you use Apple Cash --> Avoid Apple Cash scams - Apple Support
Apple 'How to identify, avoid, and report phishing' video--> https://youtu.be/SR3Z3fXXjfw
About Gift Card Scams --> About Gift Card Scams - Official Apple Support
If you are uncertain about a message and a resource provided in that message, do not click on any links in the message. Try to use an Apple resource you know is valid to independently verify what the message is claiming. Go to a support article page on apple.com and use the instructions in the article to verify though Apple itself, or use an Apple device feature such as Settings or an Apple app. To ask Apple start at this web page: Official Apple Support
Follow a shortcut to:
- How to leave a family sharing group --> Leave Family Sharing - Apple Support