Divinity need not be always visible on their faces. Their act, character build up too can represent that. Ram & Sita are different from others. The real Ram & Sita, lives in people's heart & faith. No actor's face can represent them.
Sita is explained, as Prakriti, loving Nature, can connect well to Nature compared to others.
Her lifting Shivji's dhanush, makes her stand out among others. Rishi Yagyavalkya, Vishvamitra, Parshuram sees an exceptional power in her. Even people of Mithila sees her as the reason of their wealth, prosperity. Isnt this sufficient for the bigining in the show.
Ram, is always calm, composed, full of strength. Rishi Jabali, guru Vashistha, Dashrath etc sees him as their hope. People of Ayodhya love him, for what he has done for them. Rishi Vishwamitra, saw Vishunuji's glimpse in him. Even Sumitra saw teen age Ram as Chaturbhuj Vishnuji. Those who can see his true form have known him as someone very special. May be this is sufficient to begin with for Ram
AS the show progresses, other incidents will prove, Ram & Sita are different from other humans.
They are represented as Purush & Prakriti, which is an important factor for the creation. Vishnuji is preserver. Laksmiji gives wealth, prosparity. Ram & Sita's some qualities does show them as special, Divine character. If Shivji- Parvati as Purush & Prakriti, are responsible for Creation then Vishnuji- Lakshmiji as Purush & Prakritiare responsible for preservation. Their incarnations Ram & Sita are shown having similar qualities, which does make them Divine.