Credit: Sherrie at Thai Boran Forum
I haven't been watching but maybe this will help clear up few things....
Summary:
Tao Apainooraat (nang ek's dad) will not be having any sons (the seeker said this) and he envies Chaiyechet's dad for having the deeds to have a son who's very skillful. Then Seeuma (Dear) offers the idea that their daughter should marry Chaiyachet so that he can be their son-in-law. A letter is sent to Chaiyachet's kingdom and the mom said she will take all 7 daughters and let them all be Chaiyechet's wife and one that Chaiyachet choose will be his queen (all other consorts-lower status).
A yak king came and asked the hermit for Suwincha to be his daughter since he did not have any kids (he does not have a wife too). He's a good yak who performs a lot of good deeds and merits and help the poor villagers. So he adopts nang ek.
I think from discussions at thai boran forum, he Chaiyachet proposed to have all of them as his wives.