Unfortunately, I can find little in the way of information about her on the Internet, though two sites dealing with her are here and here.
What little detail follows is gleaned from Walt Reed's "The Illustrator in America, 1860-2000." Reed and other sources I'm inclined to trust have 1878-1939 as her dates. She studied under Howard Pyle both at Drexel and in summer sessions at Chadd's Ford. Reed also notes her book illustration work and some advertising clients.
That being that, all I can do is present some examples of her work.
Stilwell was hitting the big-time around age 25.
One of Stilwell's best-known works.
It seems she borrowed the general idea ten years later for Collier's rival, the Post.
Many of her covers used children as subjects.
More leopard, this time skinned, and for Vogue.
This seems to be unsigned, but Internet sites credit her with the illustration.
A really fine illustration here.
The Russian-type costume was ill-timed, because the February Revolution (March 8-12, new calendar) occurred just after this issue was off the news stands, and Russia became more chaotic than it usually was in those years.