Quivers and Quills (Time-Traveling Twins Book 1) by Michelle Lashier

1193 is a dangerous and confusing place--especially if you're from 2009. Life has not been going as planned for 25-year-old twins Jill and Joanna Mason. Jill, an engineer and secret thrill-seeker, longs for adventure. Joanna, a wedding planner and aspiring writer, is recovering from a bad breakup. A vacation is just what they need to lighten their spirits. But neither one expects what happens next. While horseback riding through Sherwood Forest, the sisters are mysteriously transported to the 12th century where they're caught in the middle of a deadly feud. Joanna is taken captive by Guy of Gisbourne. Young women in his castle are dying, and she must identify the killer before she becomes his next victim. Meanwhile, Jill enlists Robin Hood's aid in saving Joanna. But his incompetency may cost Joanna her life. As the sisters discover hidden talents and romantic entanglements, they must decide if their destiny actually lies in the past.