Removing family member (15 year) not possible
How to delete a family member (15 year) do not get any delete button. Searched on the internet and it should be possible to delete but for some reason this is not available.
iPhone 11, iOS 15
See the Apple Support article(s) below.
If you still can't delete them, make sure their birthday is set correctly.
Note the under 13 years old restriction in the end of article below. Link to contact Apple Support at the end of the article.