Good morning DazWood1980,
Welcome to the Apple Support Communities!
I understand you have some questions about adding your mother-in-law to your Family Sharing account for the iTunes Store and Apple Music. The Terms and Conditions for Internet Services states:
"Family Sharing permits you to share eligible iTunes, App Store, Mac App Store, iBooks Store, and iCloud products with a maximum of five other members of your single family (e.g., adults and children) by allowing family members to view and download each others’ eligible products to their Associated Devices.
Family members do not need to reside at the same physical address, but their Apple ID account must be in the same iTunes Store region.
Take care.