The Trump machine claims that no nepotism laws have been violated, because Ivanka isn’t getting paid. Not directly, anyway.
However, the taxpayers are still paying for her office space, her staff, and any travel expenses related to her “position”.
It can be argued that we do the same thing for a First Lady. And that’s true. The FLOTUS is normally given an office, a staff, and travels on the taxpayer’s dime. So the solution here seems to be a simple one. Designate Ivanka as the First Lady. The FLOTUS doesn’t necessarily have to be the President’s wife. Non-wives (including daughters, daughters-in-law, and a niece) have served as FLOTUS — although Ivanka would be the first non-wife FLOTUS to serve in that position while the President’s wife is alive. But if that title went to Ivanka, Melania would have to forego any role at all (and that might be just fine with her).