Legal Question in Family Law in California

This behavior is considered parental alienation and, as you surmise, very damaging to your relationship with your child. This case may need a child custody evaluation to sort out the truth. If it is determined your ex is causing this harm, the child custody arrangements could change. Best to confer with a skilled family law attorney about your legal rights and options.

If you don't have an attorney, I would obtain one quick. From my experience, assuming the allegations are false, your brother is an easier target to get your child to say bad things about. This is just a way of easing into making it easier to manipulate the child against you, by picking someone close to you. It will eventually get around to being used against you if it isn't nipped in the bud. I have seen kids be convinced by a vindictive parent that something happened when it did not. There have been numerous studies about this type of brainwashing, and there are ways to stop it now. It is called Parental Alienation, but it is really brainwashing. Don't wait to fix it after the damage has been done.