Frank (Franz) Xavier Leyendecker (late 1870s-1924), better known as F.X. Leyendecker, was a good illustrator whose work was not as distinctive or appealing as that of his famous brother, J.C. Leyendecker.
Biographical information on F.X. is sparse. Even his year of birth is unclear: I have seen it given as 1876, 1877 and 1879. His Wikipedia entry is here and this site contains a paragraph of information and images of a number of his illustrations. The most information I could find is in this book dealing with the life and work of J.C. What follows was gleaned from it.
F.X. was born in Germany, the youngest of four siblings who moved to the United States in 1882. J.C. and F.X. traveled to Paris in 1896 for a year to study at the Académie Julian, where J.C. studied diligently while F.X. tended to focus more on drinking, drugs and carousing with other art students. After moving to New York City, F.X. was able to begin a long series of cover illustrations for Collier's magazine and also did advertising illustration. Unfortunately, his addiction problems continued and he also developed a bad attitude regarding doing commercial, rather than fine art painting. This attitude problem evolved into a kind of depression which affected his productivity which snowballed into loss of clients and commissions. In 1923 F.X. and his sister Mary moved out of J.C.'s New Rochelle mansion after a series of arguments with J.C. The next year F.X. was dead, done in one way or another due to his addictions.
Here are a few examples of his work.