Answered by Sayyidi Habib Umar bin Hafiz (may Allah protect him and benefit us by him)
What is the best way to deal with a sibling who has left the fold of Islam? What sort of relations should I keep with her?
You should pity her and show compassion to her. You should not condone what she has done but you should not cut ties with her. You should anticipate the moment in which Allah gazes upon her and returns her to the true faith. How you deal with her depends on her personality. If turning away from her would have a positive effect then do this. If not, you should treat her with gentleness from a distance until the turmoil that is in her heart has subsided to some degree. When this has occurred, she will be ready to return to Islam, Allah willing. You should pray for her abundantly.