Is the husband of the testamentary guardian considered a mahram to her niece, who is 16 years old, and is she required to wear the hijab in front of him?

A woman must observe hijab in front of her maternal aunt's husband or her paternal aunt's husband, because he is not among the mahram relatives mentioned in An-Nur/31. However, if she has suckled her brother's daughter, then her husband becomes a father to her through suckling, and he is then considered a mahram to her.