Before We Leave — Mari Collier

Before We Leave continues the scifi-western saga of Justine and Thalian descendants, living in Earth in the late 19th and early 20th century.
After Red O'Neal delivers a punishing beating to his half-brother, Daniel Hunter, for courting his sister Antoinette, she is sent to a convent until agreeing to marry a much older, wealthier man. But Antoinette manages to send a letter to her betrothed Lorenz MacDonald about her danger. After he rescues her on the journey to the convent, Lorenz's parents allow the marriage rather than lose their son.
When the neighboring ranch becomes under attack by Comanche raiders, Antoinette and her mother-in-law Anna are caught in a fight for life and death. Can their family survive the dangers of the violent Western frontier?