How can I cancel a subscription made by a family member

You need to sign into their Apple ID to cancel the subscription.

Purchases - View, change, or cancel your subscriptions.      *

Purchases - Subscriptions not billed through Apple    *

Link to contact Apple Support at the end of the article.

Purchases - the subscription that you want to cancel is not listed    

You are welcome. If she forgot the Apple ID (iCloud) password, enter the Apple (iCloud) ID, and click Forgot Password. See if she can get a Rescue or Notification email option if necessary. Link to contact Apple Support at the end of the 2nd article.

Apple ID - Manage  -     

Apple ID – Forgot Password  *