There is no guilt in Viserys' lust for you. You belong to him. Already claimed. After all, you would have been wed in the traditions of your house. Even when only a few of you remain, even when the two of you are a continent away from your ‘home’. Viserys will still uphold his duties. So when his thoughts go to what your bodies will do for the other, why should he feel shame?

Your brother has seen you bare perhaps more than yourself. He hated anyone looking at it. The servants are commanded to turn their gaze unless they wish their eyes to be plucked out. As he would undress you or take you for a bath, Viserys would examine your body to ensure it was ‘healthy’. Making passing remarks that it will only be truly healthy if your belly was swollen with child and if your tits had milk in them.

He caresses your body as he sits you on his lap. You are his sister. And he will soon marry you, fulfilling his duties. Between you and Daenery, you were his choice, without doubt. Danery was a little maggot, a filthy beast who killed his mother with her birth. But you, were his sweet sister. He hated Daenery.