Answered by Shaykh Hamza Karamali, SunniPath Academy Teacher
In the Name of Allah, Most Merciful and Compassionate
Just as you don't have to wear hijab in front of your father's brother, you also don't have to wear hijab in front of your grandfather's brother. The brother of any male in your ancestral line is like your paternal uncle, and scholars term each of these relatives a `amm (lit. "paternal uncle"). 
And Allah Most High knows best.
 `Abdullah b. `Umar b. `Abdullah Ba Makhrama, Mishkat al-Misbah fi Sharh al-`Udda wa al-Silah, al-Fasl al-thani fi akran al-nikah, al-Rukn al-khamis: al-zawjah
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