Don’t feel like you are or have ever abandoned him. Addiction changes people which is what makes it so different from many other illnesses. Sometimes the person you love isn’t even there, not enough of the time anyway because the addiction is controlling them. You must look after yourself too.
His sister knowing is a good start. Wanting to stop is a good start if it’s genuine.
As far as you personally needing people to talk to about this who won’t judge him or you, this is a great forum for that. Don’t be a stranger if you’re struggling.
You’re so very welcome