Some states have nepotism laws, some states don't. When Carter was in the White House there were objections about his wife being appointed to an honorary position. They were advised no, based on a 1967 law, but ultimately she received the position.
A law written in 1978 overrules that 1967 law, according to Politico.
Again, I'm referring you to a left leaning publication here that goes against what you're suggesting applies to the President. Fact is, he can appoint anyone he wants just like he holds the right to fire and hire foreign diplomats.
The legal opinion that cleared the way for Kushner and Ivanka Trump appointments reversed earlier advice.
www.politico.com
The opinion longtime Justice Department attorney Daniel Koffsky issued in January at the request of the incoming Trump administration concluded that another law, passed in 1978, conferredbroad authority on the president to appoint White House officials essentially overrides the earlier anti-nepotism measure.
"We believe that the President's special hiring authority [in the 1978 law] permits him to make appointments to the White House Office that the anti-nepotism statute might otherwise forbid," Koffsky wrote in the opinion sent to White House counsel Don McGahn at his request.
White House spokesman Raj Shah said the change in the law nearly three decades ago rendered the earlier opinions obsolete.
"These opinions were issued before the passage of a 1978 law specifically authorizing the President to make White House Office appointments ‘without regard to any other provision of law," Shah noted. "These legal opinions are therefore inconsistent with subsequent congressional enactments. Rather than reversing prior policy, the Administration is upholding the law as written today.”