HELP , MY NEPHEW

I live with my sister and her son he’s 2 . She works 9 to 5 . There was a Bit of time where I didn’t work so I baby sat for her until she came home . I played with my nephew, taught him things , feed him , bathe him etc . So now he’s more attached to me than he is the mother. When his mom comes home she goes straight to sleep don’t play with him etc. i decided to start playing with him because I didn’t want him to be a sad kid around adults and no one played with him . It became a routine and now even when his mom is around he prefers me. She even left for 6 months for her job in the military and I took care of him . I feel bad and it becomes awkward because who wants their child attached to someone else. His mother states why does he keep running away from me , why does he always follow you . I don’t want to cause jealousy or tension because I’d feel some way for real if that was me . The only thing I done was pay my nephew so attention and play with him so he wouldn’t feel alone. I felt sorry for him because it was like no one else would take time out with him and I had really had the opportunity to . What should I do ?