AITA for making my two friends reconcile after a racist joke incident ?

My friend, “August” 22 Female, is a woman of color. She is mixed with black and mexican heritage. My family is of norwegian and german ancestry. My younger brother, who is my best friend, “Gilbert” is 28 male. I am a 30 year old male. This past weekend, my cousin was in town so my family along with August went to historical Civil War sites in West Virginia and Virginia. August is essentially always invited to family outings, events, and vacations despite not being related because we all enjoy her company.

At one of the historical sites, my brother told August “hey, you should take a picture with this”. What he was referring to was a sign that said the following: "CAUTION TO OUTSIDERS OR NEW CITIZENS!

COLORED PEOPLE Of Virginia by law

* Cannot Vote

* Cannot be taught to read or write

* Slaves cannot earn an income nor purchase another slave - It is the duty of the town sergeant to enforce 10:00 P.M. curfew for all slaves and free blacks. Any negro caught out after curfew, will be arrested & can whipped ten lashes. - No more than two free blacks or slaves can gather on the street at any given time. -Slaves that escaped to a free state are pursued and brought back to bondage.

By the ORDER OF THE MAYOR & ALDERMAN"

My family and I didn’t react to the sign or Gilbert’s statement. August didn’t see the sign because she was observing other artifacts, so there wasn’t a reaction. By the time she saw the sign my family and I were moving onto the next exhibition. Afterwards, August isolated herself and was essentially silent. She only spoke to my parents and visiting family. Later that evening, August and I had an argument where she stayed I didn’t “have her back” like I usually do. Additionally, August expressed that she felt ostracized, humiliated, and embarrassed by my brother, Gilbert’s actions. I explained to her that if she felt so deeply about what my brother said she should’ve spoken up sooner, but August replied that her being the only person of color in the room with my white family made it feel impossible to do such. August and I buried the hatchet but she refuses to acknowledge or speak to my brother until he apologizes. I spoke to my brother (with August’s blessing) to explain to him why what he did was hurtful and how she wanted an apology. My brother has yet to apologize since our conversation and August is still being cold towards him. I yelled at August and told her that she should stop holding this grudge against Gilbert, and make an effort to make up with my brother since we’re going on a three week vacation abroad soon so I don’t want to be the middle man between their squabble. August called me an asshole for siding with my brother and continuing to provide (what she calls) excuses for my brother. AITA?