You do not say if this younger daughter is a minor child or not. If she is not a "child" according to Apple, have her leave the first family group. How to leave a family sharing group --> Leave Family Sharing - Apple Support
If she is a minor child, normally transferring to another family is a process involving both group organizers. This would entail the older daughter going back into her account and doing it Apple's way. See this article about how to move a child to another group using Family Sharing - Move a child to another group using Family Sharing - Apple Support
If the account is truly completely inaccessible you will have to see if there is anything Apple can do to help you with this. It may ultimately entail your younger daughter abandoning her old account (and everything in it) and setting up a new one with the new group. Follow the instructions in this support article to create an Apple ID for your child - Create an Apple ID for your child - Apple Support
Refer to this document for ways to contact Apple ➞ Choose your country or region - Official Apple Support
Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service by telephone ➞ Contact Apple for support and service - Apple Support
