My sis-in-law sent her son to come live with us, saying she wasn't going to support him in anyway. What are our options for filing for custody?

My fiancee's sister is an alcoholic and living with her bf. They have broken up a couple of times recently leaving my sis-in-law and her 2 kids homeless. Understandably, her 17 yo son is having problems in school now because of all the chaos in his life. Both kids have missed a lot of school already this year. In the last couple of weeks, my sis-in-law has returned to her bf's house with her kids. We know it's a matter of time before she is back out on the streets. On 11/16, our nephew called me crying because he, his mom and her bf got into a huge fight and they were saying they were going to bring him up to stay at our house for good the next night. He has spent a lot of time here already. We called my sis-in-law and she said our nephew wanted to come live with us and so they were going to bring him the next night and it would be for good. He could never return and she wasn't going to support him in any way. She would give him clothes or something for X-mas, but that was all.