It is a loss for all of us - the alcoholics, and their families - when alcohol takes over and we can no longer look forward to sharing a part of our lives with friends and family because of it.
I'm sorry for your loss. You did the best you could with your frank discussions with your sister-in-law and your niece, and I imagine that they now feel more aware and supported than they would have been had you not visited. And, you are right, there is not much more you can do except support them from a distance. And that is a loss for you.
Hope you find other golf partners and can still plan a spring getaway.
ShootingStar1