Best Friend maybe a creep or his Brother is crazy

Ok. So my friend has a brother that has 3 children, 2 girls and a boy all under 10. Brother is very anti establishment and keeps his kids from school to control what they learn. Hes anti government, anti police, strongly right wing socially. He alsoncut off contact with his parents because they were too leftist. These kids do not get much socialization beyond each other. Brother has been getting my friend to teach the kids during the day for the last few weeks.

I got a message from Brother last night stating my friend has been grooming the oldest girl for last 2 weeks and had touched her chest, kissed her on the lips. He said his brother, my friend, is a creep and was telling me this because he wanted to ruin his life since his brother had attacked his family. I called Brother to find out more details and was told he confronted my friend about it and that friend eventually confessed to him that he did it and it wasn't just the girl lying.

Ok so I then contacted my friend, let him know I talked to his brother and wanted to hear his side of story. He said that his brother was crazy and has been attacking people all the time, will say stuff like this about people for no reason. Says he talks shit about everyone, including me. He also said he physically attacked their father today, didn't know why. Friend admitted to hugging and kissing nieces but no more. Asked why oldest would say otherwise and his reponse is that she would say whatever Brother told her too. He was upset and said he's sorry I'm in the middle of it and hoped it didn't ruin our friendship.

Friend is supposed to stand with me at my wedding later this year. I myself have been abused as a child by an uncle and just am feeling ridiculously triggered by this all. I want to believe my friend, but do I only want to do that because its easiest. Is there such a thing as an objective observer that can see through the bs?