boundaries I guess

Emeline
Hi, so I moved in with my boyfriend and his mother a few months ago. His mother watches his nephew everyday and as a result the nephew (G) and I have become close. Over this past month his sister has decided that to save money she and her son (G) will be moving back home. His sister is not a good mother. Far from it, either the grandmother, my boyfriend, or I watch this little boy every day after he gets home from preschool while she hangs out with her boyfriend or ignores him entirely. He's three years old and very developmentally behind, due to the fact that his mom ignores him mostly his grandmother is ok and his uncle (my boyfriend) is at work all the time so before I moved in nobody worked with him on his ABCs or his numbers so his preschool teacher is sending home work and the like because he's nearly four and cannot consistently count to ten. He's very smart and loves to be read to and I try to help but I reach the point where I don't want to overstep my bounds. I'm not his mom, I'm not related to him, I haven't even been dating his uncle all that long. But he has attached himself to me at the hip, my boyfriend says it's because he's trying to find that maternal figure. And I don't disagree I just don't want to make an enemy with his mother who doesn't really like me because her son prefers me to her. It's not really a shock because he can tell that she doesn't really want him and he was really born to try to keep her ex. Like I said he's very smart. But how do I go about this. I don't want to ruin my relationship with this child. I want to be a kindergarten teacher eventually this can only help. However I also share a living space with his mom so I don't want to make an enemy. And I'm not looking to be his mom. She gave birth to him. Not me and I'm by no means trying to take that from her. I just want him to be able to succeed as well, he's very smart he's not getting to live up to his full potential if he's being ignored and shoved off on junk food and tv. He loves to read and write and draw and learn and nobody seems to really want to help with that but me. My boyfriend tries but he works overnights so he's asleep when g isn't at school.