Your husband is clearly sexually attracted to your fifteen year old daughter. The reality is that we humans are filled with all kinds of animalistic desires and urges, both sexual and aggressive. The reason the incest taboo was created was precisely because it is normal to have feelings for family members. Acting on these desires would destroy the family bond, so the incest taboo prohibits people from acting on these normal urges for family members and thereby serves to insure that the human race doesn’t kill itself off. Your husband can’t help his natural feelings of attraction for your daughter. So long as he behaves properly and doesn’t act on his urges, everything is fine.