Hi, I'd like to help you with your question.
It sounds like your sister either has something serious going on medically or she is unable to handle stress very well and becomes physically and emotionally abusive to her children as a result.
It is important that you consider contacting child services. It's hard to think about turning your sister in, but it is necessary in order to protect your nephews. You can petition to care for them if it becomes necessary. But getting those children protected and out of that environment is a priority. You can also contact child services anonymously if you do not want anyone to know you called. They are obligated to investigate any claims of abuse.
The other option is to ask your sister if she needs a break and offer to take the children in for a while. That way, you get a chance to stop the abuse, assess the situation and decide how to handle it, and your sister can take time to find out what is going on with her.
Here is a link to therapists in your area:
Gina Lynn Patterson, PsyD
I am a licensed psychologist who is also the Clinical Director of Every Woman's House, a private non-profit agency that specializes in family violence and sexual abuse/assault. I specialize in trauma and family issues. I also conduct psychological assessments in these areas. At my agency there is also shelter and advocacy available if needed.
Here is another link for information about child abuse and how you can help your sister and nephews:
Let me know if there is any other way I can help,