@AmiB,
Do you think your aunt is willing to take the other kids in?
CPS typically wants children to stay with family - so if your aunt is responsible and she is willing that is the best case.
My reasoning is it should not be your burden in a sense to beat up your mother if she abuses your siblings. You should not have to be their defender - that does not mean you should not report her if you feel comfortable doing so.
I would side on talking with a school counselor. Most are very good. Do you know one that you trust? That is the ideal situation. S/he would be able to guide you and help you as well on your options and in many cases someone outside of school that can help you - like a lawyer or a social case worker.
The one good thing your siblings have is you - and even if for some reason your aunt is not able to care for them, you will be able to keep an eye out if whoever CPS would place your sibling with you can make sure they are in a good place. There are many good families that take in children like this situation. I personally know one local family here that had taken in a little girl and then later her brother. They are both teachers, there other children had all just graduated high school and they were in the perfect position to do this. They have since adopted these children.
Yeah there is bad there - and CPS is often times overworked. But if they have someone like you to speak up for them - it could be a good thing.
On the flip side - if your mom and dad have issues, CPS could possibly help them as well. If your mom and dad really want to have their children, there is likely counseling and other services that could help them and teach them why what they are doing is wrong and hurtful.
But if they stay where they are now - they are almost guaranteed to have continued abuse.