Complex sentence: They have to deal with a new coach, and new teammates--including a new upcoming gymnast, Wendy, who has more energy to train harder, since she is younger than the more experienced Seniors.
Split into simpler sentences: They have to deal with a new coach, and new teammates. They must deal with a new upcoming gymnast, Wendy, who has more energy to train harder with the advantage of being younger than the more experienced Seniors.