words - First use of the term "Taharat HaMischpacha "

When and by whom (and why) was the phrase "טהרת המשפחה" ("family purity") first used to describe the halachot relating to niddah?

Answer

There's a Sefer by that name from 90 years ago - תרפג/1923.

A search of HebrewBooks.org seems to show that the term was not used [much] earlier.

A search of Toras Emeth Software indicates that it's not used in any of the classics (Mishna, Gemara, Rambam, Shulchan Aruch) and the earliest it finds is from the קיצור ש''ע ילקוט יוסף - the 2nd half of the 20th century.