Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.

 

In the Name of Allah, Most Gracious, Most Merciful.

 

All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.

 

Your husband is wrong in treating the girl who has attained puberty as if she is his blood relation. No. she is not. There is no marriage taboo between them. That is clear from the Qur’an. So, she has to observe the rules of hijab in his presence, and he is not allowed to be alone with her. She is not different from any other mature woman without close blood relationship.

The categories of women we are not allowed to marry are listed here in the following verse:

 

“Forbidden to you are your mothers, and your daughters, and your sisters, and your aunts paternal and maternal, and a brother’s daughters, and a sister’s daughters; and your milk-mothers, and your milk-sisters; and the mothers of your wives; and your step-daughters – who are your foster children – born of your wives with whom you have consummated your marriage; but if you have not consummated your marriage, you will incur no sin [by marrying their daughters]; and [forbidden to you are] the spouses of the sons who have sprung from your loins; and [you are forbidden] to have two sisters [as your wives] at one and the same time – but what is past is past: for, behold, God is indeed much-forgiving, a dispenser of grace.” (An-Nisaa’ 4:23)

 

So, the girl you mentioned is not one of them; as such, she has to observe proper rules of hijab or modest attire in front of your husband.

 

Allah Almighty knows best.