Sally Hemings was 9 years old at the time of Martha Jefferson’s death. Martha Jefferson’s father (John Wayles) was also the father of Sally Hemings. That made Sally Mrs. Jefferson’s half sister.
She also was something like 7/8 white and looked just like Martha Jefferson’s twin.
During my time a The University, it was said that Mr. Jefferson transferred his love from his deceased wife, Martha, to Sally.
Also, Martha Randolph Jefferson, Mr. Jefferson’s daughter, acted as First Lady during his service as POTUS, so Ivanka’s role has historic precedent.