Does the expiation for ẓihār become obligatory for one who said to his wife: "By God Almighty, I will not come to your side ever again, and let us deal with each other as sisters without problems," due to her repeated refusal of intercourse?

The phrase "Let's be sisters" does not constitute Dhihar unless accompanied by intention. As for swearing an oath, it is considered Ila' and it is not permissible to abstain from one's wife because of it for more than four months. If a man fears that his wife's aversion to intercourse is due to magic, he must seek treatment for it.