Kara-ann—
Based on your question, I presume your sister was raising in the faith and left for the Witnesses. In these cases, there is little one can do unless that person really wants to engage you and ask you questions. In that case, the best tactic is to ask them to show you their "proofs" from the Bible for this, since it is so contrary to what all Christians believe; then you will know how to talk with her.
The "sleeping" in the Old Testament refers to Hades or Sheol, the underworld, where the dead had a less than vital existence. In the New Testament, the dead who have "fallen asleep" in Christ, are those who have died in the body, and their body awaits Resurrection. As for the immortal soul, only God is eternal. Even pagans believed this, like Plato, for instance. Man seems to know this by nature, reflecting upon his mind and will. See this link for an overview, which does include Old Testament Scriptures:
Immortality from New Advent web site
The New Testament speaks often of the dead being with the Lord, even those who "sleep under the altar" in the Book of Revelation. (Revelation 6:9-11) Their souls are with the Lord.
The soul cannot die because it is immaterial and cannot "come apart", since spirit has no parts.
It cannot corrupt, even in hell. It does need the body for its full expression, however exists in an imperfect state until the General Resurrection. However, the souls of the just are able to exist, alive, blessed, and happy, because they are ordered in some way to the Body of Christ.
I hope this helps. One or more of my colleagues should be able to provide more of a Scriptural basis, if that is what you are looking for.
Mary Ann
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