I’m so sorry your family is going through such a hard time. Your sister has had more than her share of problems in life. Her conversion may have been her attempt to find some clear rules for living her life that would give her a structure and some safety. I’m glad you can support her in that choice.
The kids need you and your family. A mother’s attempt to suicide, as you pointed out, is traumatizing. I hope you are getting some therapy for them and for yourself. You and your parents need some guidance about how to handle the situation. The kids need you to handle these events well. Meanwhile, therapy that includes your sister, you, and your parents is more likely to be effective than sending her off for individual treatment. She needs to know that you are all there for her. You all need to know how best to help her.
From what you wrote, I think your sister is worn out and the rest of the family is at their wits’ ends, having tried everything they already know how to do. Therapy can provide a safe place where everyone can talk and be heard. An experienced therapist will be able to provide you with referrals to resources you may not know about as well as some practical advice about how to move forward.
I wish you well.