Complex sentence: They try to make her have fun and teach her to fly, but she fails, because she does not believe in fairies, leaving Tinker Bell to die until Peter tells the boys they have to make her part of the group.
Split into simpler sentences: They try to make her have fun and teach her to fly, but to no avail because she does not believe in fairies. This leaves Tinker Bell quite weakened and on the verge of nonexistence until Peter tells the boys they have to make her part of the group.