this seems to really be a question whose answer depends on the culture they live in. The "five good emperors" Hadrian, Trajan, Nerva, Marcus Aurillius and Antoninus pious all adopted their heirs and such adoptions were more common in Roman times than say in Elizabethian england.

Would it have been more preferable for her to 'adopt'

that's tricky because this move needs to be seen as legitimate