My 19 years old sister is married now. She is very bad; she is naked daily in front of her husband at night. Her husband is very good: he didn't like that. So please inform me if its Halal, Haram, Makhroh.

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The woman is allowed to be naked in front of her husband if nobody else could see her. The Prophet said: "There is no Awrah (private parts to be screened) between you (the man) and your wife …".
So, there is nothing in this act that one should deny. If her husband does not like this act because he thinks it is contrary to Sharia, he should know that it conforms to Sharia rules. But if he just does not like seeing her naked for some other reasons, then she is supposed to leave whatever her husband does not like as that is better for their mutual understanding.
Allah knows best.