The only sin God cannot forgive is the sin of rejecting the Holy Spirit's witness to Jesus Christ and His offer of forgiveness. Only when we reject God will He reject us. This alone is the unforgivable "blasphemy against the Spirit" of which Jesus spoke (Matthew 12:31).
God understands your heartache - and I want to assure you that He also knew all about your sister's struggles. Medical science has made many advances in helping us understand emotional problems, but there's still much we don't understand. Take confidence in your sister's faith in Christ, and rejoice that she is now beyond the pains and sorrows of this world.
I don't want to be misunderstood, however. Suicide is extremely serious and tragic in God's eyes, and if someone who is reading this is contemplating suicide, I beg of you to reconsider and seek help for whatever your problem may be. God loves you - whether you believe it or not - and He does not want you to end your life. Satan does, however - and you must not listen to him.
Instead, put your faith and hope in Christ and His love for you. You are never alone if you know Him, because nothing "will be able to separate us from the love of God that is in Christ Jesus our Lord" (Romans 8:39).