Children of 3 year old do not try out of the blue to put their penis in their baby bothers butt ..however it may be a good idea to run it by your doctor or some one who may have more knowledge
Wow that can not possible be her answer to everything. In this day and age it is a good question though. But according to child psychology she is a bit wrong about children just suddenly doing things like this on their own. God did not show adam and eve how to experiment with their sexuality. It is normal for children under 5 to be this experimental. It seems unusual to us and possibly scary. Do not worry about your child picking a sexual pref. yet. So when you ask your child were they are learning this be completely calm like it is not a big deal. If this is really the issue of learned behavior then do what you have to do. Then explain that both you and your brother can get hurt and you are not to do that again.
I would ask him where he had seen the behavior, children do not all of a sudden do things like this they copy ...