If you've got evidence that this woman is beating her kid, then you absolutely did the right thing by alerting the authorities. I have no idea why your therapist tried to make excuses for her, but there is no excuse good enough for that kind of thing. It's not even a matter of fault, as far as I'm concerned. (For example, if your step-sister had some sort of brain injury and was not responsible for her actions, you'd still have to protect the children because the cause doesn't matter. If a kid is getting beat, they're getting beat.)

For what it's worth, you have my support. No matter what happens now, please know that you've done the right thing.