First let me say how sorry I am to hear about your sister. Such loss is hard to understand and even harder to bear.
I have some understanding of your situation as I have someone close to me who has bipolar and I live in fear of her getting to a point where it is far too much for her. I have lived with it now for more than four years and have daily cried out to God to keep her safe and heal her. What God has taught me through this is that he is in control and he loves her more than I could possibly imagine. He is as much concerned for her as he is for me and while I would not have chosen this path for her, God has and he does not make mistakes. This is the path God wants her to follow and faith demands that I trust God in this even though I don’t know what the outcome is. She may in the end take her own life and as tragic and painful as this would be (and believe me I pray desperately that it would not happen), even this is not beyond God’s control and I have to accept it and let God be God!
After all this is what faith is - trusting God and trusting that he knows what he is doing - and it is in situations like this that our faith is tested to the limits. This is the intent of Psalm 46 where the psalmist speaks of trusting God even though the world passes away. In the midst of everything, God is his refuge and strength. The most amazing line in the psalm - Be still and know that I am God (vs 10) - is a call to us in the midst of our pain, anguish and suffering to stop, be still and remember that God is in control and nothing can thwart his plans nor his love for his people. He will be exalted and his people with him.
And this is the same faith you need to show in your situation for I’m sorry to say that I cannot give you the assurance you seek. I do not know what is in another person’s heart only God does So I cannot know for certain whether your sister (or anyone) is in heaven or not - that is between her and God. But I can tell you this:
God is just - he will not judge her by anything that she could not know or was not able to understand. He knows her heart and will do what is right.
God is merciful - all it would require is faith the size of a mustard seed.
God is loving - he will bring good out of something even as hard and painful as this.
These are the things that you can be certain of and can put your faith in. If there is a mustard seed of faith, God will save her. You may not know whether she is in heaven or not but you can know that God, who has her in his hand, is faithful and just and will do what is right.
I’m sorry that I cannot be more helpful but we are called to be people of faith who take God at his word and sometimes that is all we have to hold onto. And while this might seem like nothing (especially to those who do not understand) this word is stronger than anything that I know and will outlast time itself. So hang in there, grieve for your sister but remember the hope that you have in Jesus.
Your friend in Christ