I think and hope the boys will be or have been thoroughly interviewed by professionals.No matter what happened to this little girl the brothers are going to suffer for years to come.
"I told the boys to watch Summer". If it was two minutes or two hours it was put on them, and that's when she was gone. Mom can explain she walked her up to the porch. Made sure the boys were there. Tells them watch her I'll be right back. Says she is in two minutes. She calls down. She's gone.
God Bless the child that's got their own.
They should feel no guilt imo.