You need to do something NOW to protect your children. This is abuse. Doesn't matter if its from an adult or a child. Abuse is abuse. Your 2 1/2 year old is showing signs of abuse already and yes he will probably repeat these actions on his sister.
For the two older boys to be acting this way, they must have been abused sometime or other by someone.
You need to protect your children. Why isn't your husband more concerned? They are his children also.
Your children are your first responsibility. If your husband will not discipline the older boys, you need to take steps to make sure your children are protected. If that means leaving, so be it. You will be looking behind your shoulder at all times. It will only get worse.