Arthur likes to think, in his more reflective moments, that if his mother had lived he would have liked to have a relationship with her like the one Merlin has with Hunith.
Merlin doesn't lie to his mother—he sees no reason for it. He comes to her with questions and concerns, or even when he just needs comforting. And Hunith is there with advice or an amusing story or a warm hug that lasts as long as Merlin needs it to or whatever the situation calls for.
She's strong, as well. She's clearly the village leader, and she is quick-thinking and discerning.
Merlin once admitted to Arthur that when he was little, he wanted to be just like his mother when he grew up.
Arthur half-expected Hunith to give him a stern talk the way Gwen had, about how Merlin is wonderful and loves with his whole being and would give his soul to Arthur if he could and that if Arthur hurts him in any way retribution will be swift in coming but will take a long and painful time leaving, but Hunith doesn't do that at all. Instead she hugs him, and smiles at him with warm eyes, and tells him, "Welcome to the family, Arthur."
And after a while 'Arthur' becomes 'my boy' which becomes 'my son'.
Arthur doesn't lie to Hunith—he sees no reason for it. He comes to her with questions and concerns, or even when he just needs comforting. And Hunith is there with advice or an amusing story or a warm hug that lasts as long as Arthur needs it to or whatever the situation calls for.
And Arthur slowly realizes that his birth mother may have died, but that doesn't mean he's motherless.
In fact, he has a very wonderful mother. And, like Merlin, he kind of wants to be like her when he grows up.