They specifically say 2 adults can board with a child in family boarding - other minor siblings obviously can as well.
No, they specifically say AN ADULT:
https://www.southwest.com/html/generated/help/faqs/boarding_the_plane_faq.htmlAn adult traveling with a child six years old or younger may board during Family Boarding, which occurs after the “A” group has boarded and before the “B” group begins boarding. If the child and the adult are both holding an “A” boarding pass, they should board in their assigned boarding position.