He sponsored her when she was an orphan and he does not want her to wear hijaab in front of him

I am a female. and, me and my brother are both adopted. We were brought into the house when we were only a few days old. We lived our lives with our adoptive parents, not knowing our conditon. After practicing Islam, I came to know that i was adopted. My age is now XXX. I lived with my parents until 18, then i married. My husband told me that i must start to wear hijaab in front of my father, and that he is not considered a mahram. We aksed, and someone said that we are Rabaa'b that Allah mentioned in the Qur'an and that hijaab need not be done. We did not feel comfortable and after asking further, we came to believe that He is not my Mahram, and that I must do hijab. My adoptive father and mother are both very emotional people. Whenever i mentioned adoption before, they imediately started to cry and could not talk. This happens all the time.
Me and my husband were both scared that they would take this very badly, so we decided that we would be subtle in the way we took this issue. When i visited my adoptive parents last, i covered my hair with a type of pakistani scarf which is not transparent, giving the excuse that many people work around the house. Also, i did not wear a jilbaab, but wore lose clothes, Qamees and Sharwaal. Also i ate at the table, and also freely talked to my adoptive father.
In front of all other non mahrams, i fully cover myself in black, even cover my eyes, and do not talk to men without necessity.
Allah knows that the reason i am not doing full hijab is because of my adoptive parents would react. They are old Father XXX, mother XXX, and they are weak in their imaan in that they would not be able to understand nor accept this ruling in Islam. My father tries to pray, but misses it much, and my mother prays regularly, though missing some and making them up.What am I supposed