The fact that you had to say ‘by law’ nullifies the natural part of natural born.
By the immigration act passed by the first Congress a child born to US citizens overseas ‘was considered’ a ‘natural born Citizen’ (note that the wording, capitalization exactly matched Article II, Section 1. No hyphen, capital C). So the intent of that act was just as you describe to grant, via law, natural born status to those born overseas to citizen parents (note that is it is plural).
Unfortunately for your sister, assuming she has plans to make a run on the US Presidency, the next version of the immigration dropped that passage. And thus, there is no law and has not been for 200 years that declares a person born to US Citizens (such as John McCain) a natural born Citizen according to Article II, Section 1.
Notice that the law even acknowleges that a person born overseas to US Citizens is not a natural born Citizen. The law made them legally equal with declearing them an actual natural born Citizen. Those in the category were CONSIDERED AS. This is because natural born Citizen is not a legal construct. It is a construct that is considered part of natural law and beyond the controls of ordinary man-made law. So the founders never attempted to define something they considered a non-legal issue. And when they knew they wanted sons borh overseas eligible for the Presidency they did make make a law to have them CONSIDERED AS natural born Citizens - because they knew - they were not.
But then they removed the passage. Likely because they realized the had illegally attempted to alter the base Constitution in a non-Constitutional manner.
Regardless of what the philosophical discussion were of the day.
Title 18 Section 1401 gives legal meaning to the Constitution’s words. It clarifies what the Congress considers natural born. You can try to claim the law violates the Constitution but the courts have repeatedly skirted the issue and have allowed the law to stand.
Unless you get that law overturned, all arguments to the contrary are moot points.
I believe Zero was not born here, and if he was, his mother had spent too much time overseas and not enough time here to qualify to give birth to a US citizen. I am foggy on this law but I believe she had been out of the country for 5 years, and as a 19 year old, needed to be here at least 1 full year before any offspring could be given a US birth certificate.
That is an argument you might just win.
I also believe he has applied for student program, grants, admissions, etc., as a foreign person and has used multiple social security numbers, which gives me cause for believing he does not even have a birth certificate to show.