Re: My older sister invited me to her Apple Family but I can't join
Seems the Family Sharing organiser can only invite five other people to join,
and
everyone should have their own Apple Account.
Wondering whether Apple Support's online article (link below) can help
How to set up Family Sharing on your iPhone, iPad, or Mac - Apple Support
Eg: says:
"If you can't accept the invitation
If you can't accept the invitation, check if someone else joined a Family Sharing group with your Apple Account. Remember, you can only join one Family Sharing group at a time, and you can only switch to a different Family Sharing group once per year."
There's a lot more info. in the above article, if the above info. hasn't helped.
Also you may want to check you aren't still linked to the Group that you set up.
See: How to leave or remove a member from a Family Sharing group - Apple Support