My rights as a live in girlfriend?

My boyfriend and I have a child together and he has recently ( about a year ago) acquired full custody of his 2 sons, as their mother is mentally and physically abusive and unstable. I cook for them, do the laundry pick them up from school, help with homework-everything a mom does. My boyfriend is leaving the country for 2 weeks in the summer and we do NOT want the boys staying with their mom, but we are not sure that legally they can stay with me as I am not a legal guardian. My name is on our residence too, and I do maintain to keep up the visitation schedule with their mom, so long as I feel that they are safe. She has threatened to kill me and my child several times. What can I do to make sure that there can be no recourse for them staying with me? They want to stay with me-their ages are 11 and 13.