It is quite likely that a shared use by mutual agreement does NOT rise to a prescriptive easement .

It is rather likely that a shared meter runs afoul of public utility regulation and or local codes...but it may not be necessary to dig thru same.

As a practical matter you may hold the trump card ..IF Sis fails to reach a written shared agreement and $$ to your satisfaction ...Plan B if she doesn't like your way is to cut the line on your property?

So far nobody has addressed an easement of necessity ...and that's speculative for now.