She has nothing to forgive you for. Forgiveness is not something that another person bestows on another; forgiveness is a healing that each individual must give to themselves. Forgiveness has nothing to do with another person; if you believe that, than you are stripping yourself of the inner power and strength that is your God given birth right.
Only you Christine can release yourself from the prison of guilt. No one can do this for you, not your sister, not a doctor, not even God, you are a trapped prisoner wanting out of yourself imposed cell and you don’t even realize you are holding the key to your release.
Your sister does not blame you, nor judge you in any way not even remotely, I promise you that. Her demons were just that: hers! Had you known her reality you would have acted differently perhaps, but you did not. Accept that, love your sister enough to forgive yourself for not being all-seeing and perfect and understand that your negative thoughts and hurt feelings for yourself affect your sister in spirit. When you hurt she hurts, when you are happy so is she, I promise you that is how life in spirit works. You were never meant to prevent her death, believe or not, but accept that or you will piece by piece self-destruct. If the roles were reversed would you want her feeling as you do knowing how deeply it hurts you in spirit?
She is safe and still with you. You can be angry with her for how she died and why she died and all that, but then push through it and file it under G because it does not help you. Talk to her!