OPINION:
You’re both adults.
You chose God; she did not.
You will pray for your sister.
She obviously doesn’t know how to pray to God.
You can send her a Bible with a maybe a special
book on how to get out of the dark hole she apparently
crawled in to.
She may or may not thank you.
YOU will pray for your sister.
That’s it.
The parable of the man receiving visitors in the middle of the night, comes to mind. He asks his neighbor for bread to give them. It is custom to feed visitors, but everyone is asleep. He says go away. The neighbor is God, the asker is you, and the visitors are people that need Bread( Jesus). He asks, and asks, until the neighbor finally gives him Bread to make him go away. The fervent prayer of a servant is very powerful, but God makes the decisions and no matter what we think, He is the Sovereign. It's so hard to accept, but some will never be saved. They don't belong to you, they belong to God. He is the potter and he can break them and throw them away if He chooses. For some reason we can accept a person at work being lost, but we cannot accept a family member. We are all equal in God's eyes. God rejoices just as much if a stranger repents as a family member.