custody of illegitimate spurious child from minor parents

I am 22 years old and I have a nephew from my 18 year old sister. She had him when she was still 14. She and his boyfriend (who was 18 at the time of birth) isn't married. Unemployed and currently studying, the mom lives in the university dormitory and only sees the child once a week. The father - only an HS graduate - doesn't have a stable job and also isn't financially capable to take care of the child. The child is now living with me with my support and from that of my parents (both living in the States). Sometimes when the mother doesn't have money left from her allowance or the time (exam week) to take care of the child she doesn't follow her weekly schedule of getting the child from me and forces me instead to take care of the child. Can I legally adopt the child or gain parental authority/custody over him?

are the parents amenable to your plan?

Unfortunately no, but considering the lack of financial capabilities of both parents I think that it's best that the child stays with me.

the child can stay with you, but only with the mother's consent which she can withdraw anytime. you can make a list of your expenses and ask her to pay,but don't use the child as collateral.

you can try to adopt the child, but it will be better to have consent of the parents. your better economic condition is not sufficient to over ride her parental rights.