When you talk about the idea of “believer's sleep” two things come to mind:
1. It sounds like you have studied the scriptures and have a good idea of what happens when a believer dies. Let me add the story of Lazarus in John 11:11. When Jesus tells His disciples about the death of Lazarus, He says “Our friend Lazarus has fallen asleep, but I go to awake him out of sleep.” Notice Jesus does not here say, “The soul of Lazarus is sleeping,” nor, in fact does any passage in Scripture say that the soul of a person is sleeping or unconscious (a statement that would be necessary to prove the doctrine of soul sleep). Rather Jesus simply says that Lazarus has fallen asleep. Then John explains, “Now Jesus had spoken of his death, but they thought that he meant taking rest in sleep. Then Jesus told them plainly, ‘Lazarus is dead’” (John 11:12-13). Other passages that speak about people sleeping when they die are likewise to be interpreted as simply a metaphorical expression to teach that death is temporary.
I took the above explanation from Systematic Theology by Wayne Grudem, InterVarsity Press.
The bottom line is sleep is a metaphor just like many of the other things Jesus talked about... “I am the true vine”, “the fields are white unto harvest”, etc.
2. You say you found “no comfort and peace” in the message that your sister is sleeping. I, like you, understand the scripture to say that we are immediately present with the Lord when we die.
But let’s just say we are mistaken and we go into a state of total unawareness until the rapture. Then we are raised to spend the rest of eternity with the Lord. If I may be so direct, I must ask this question: what difference does it make? In either case we spend all of eternity with our Lord and King. In either case we are aware only of immediate presence with the Lord after we die.
In my mind this is not a central doctrinal issue. You can trust a wonderful loving God for your sister. She is in the care of the One True God who loves her more than you do. I think most Adventists would agree with that.