It would be all but impossible for Apple to offer this since every country has this own unique set of copyright laws. Even different companies may own different copyrighted tracks in different countries. Copyright laws far predates the global market.
Couldn't they just issue localised subscriptions, e.g. if OP's overseas relatives were in the UK they could have a UK Apple Music account 'billed' to the US family subscription.
I think a more fundamental reason is Apple almost never allows delivery of services in an alternative country to where it is billed.
If they allowed such cross-pollination of transactions then everyone in the UK, where a Music subscription is £9.99 per month would sign up a US account where it is $9.99 and save a few dollars a month.