Welcome, Happy. Sounds like a very complicated situation. Are there any other family members in the area that may be able to help care for your nephew? Does your father help care for him when he is home? I'm not sure that calling child protective services would make much difference in this situation.
From what you have said, it seems that you know that you need to take care of your self first and foremost. I think most of the posters here agree that extends to helping care for the children in the given situation, to the extent that you are able to do so. Being so far from them there is little you can physically do to support him. Can you call him or skype with him regularly? Conversation with a 5 year old is limited, but it sounds like he could use a positive male role model in his life and it may help to give you a glimpse of what he is dealing with and help you determine how you may be able to help further if you want to.
I'm sure other posters with more applicable experience will be along to respond. I'm sorry for what you have been through and what has brought you here, though I believe you will find some of the support you need.