Sarah Mills was not the mother of this Ninian Magruder:
Ninian Magruder
There were several Ninian Magruders and perhaps she was the mother of another one, but I'm in the process of adding some screenshots and links of multiple documents showing that the Ninian who was married to Elizabeth Brewer was the son of Col. Ninian Magruder and his wife Sarah Beall.
I may not get it all added to the respective profiles tonight, because it's late and I'm still adding the screenshots to the profiles in my Ancestry tree to get my lines correct there. I like to mirror my work there with what is connected here on Geni to make sure everything matches up, which is how I came to realize that this is the wrong Sarah for this particular Magruder family.
I'm unable to copy and paste from documents, which is why I'm having to make screenshots to share.