Is 12 too young to explain mental illness?

Me, my 12 year old daughter, and my husband were at Walgreens to pick up some medicine for my daughter's ear infection and we saw my husband's older brother. He was yelling at the clerk that he wants to kill women and he hates women. My daughter was very confused. My husband went to intervene and offer his brother a ride home. My daughter keeps asking about it. So my BIL is extremely mentally ill. He's bipolar and schizophrenic and would randomly stop taking his medication. He has a 6 year old son and he randomly decided to stop taking his medication and he didn't abuse his son but wasn't stable to have him. So he was taken away but CPS, which were two women caseworkers who took his son away. Ever since he hasn't been taking his medication at all and we hadn't really seen him. My husband's mom told us he was saying things like that because two women took his son away. We found out when my husband went back to Walgreens for something what happened. His brother was trying to get a picture of his son printed out and needed help getting the picture from his phone and it seems the clerk was snarky about it and he tried to pay for the picture with pennies and she said she wasn't counting them and started laughing at him and that seems to be what set him off. My husband just paid himself so his brother can get the picture of his son that he wanted. My daughter keeps asking why he was saying that and what's wrong with him. Is 12 too young to understand?