So here is my opinion about the relationship between Tanya and Eunseom, Saya, Tagon, I think at first, the writers planned to have a love triangle between Eunseom, Tanya and Tagon (so we had a rumor of Tagon x Tanya before the drama aired), but then they wanted a surprise plus they only had a limited time (18 EPs to go) so they let Song Joong Ki played both Eunseom and Saya and let both brothers loved Tanya. So at the end of part 1, they didn't let Tagon develop any romantic feeling for Tanya, while they hinted us about the (love) triangle between two brothers and Tanya (to be honest, it's not just a hint anymore, they've already clarified that Saya will love Tanya in the relationship chart of part 2). The love Saya has for Tanya is less complicated to explain because we've already known that Eunseom could see Saya's world so of course, Saya could do the same, and Tanya is Eunseom's world, she is "his friend, his mom and his lover" (according to SJK), so Saya loves Tanya because he can feel that his brother loves this girl dearly. In this way, Tanya has nurtured two brothers at the same time without knowing it. So basically the three have a complicated, indefinable and spiritual bond, and not to mention that the prophecy has already sealed their fate since the beginning, and somehow, the bond makes their love triangle less awkward and forced.