All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The jurists stated the order of the relatives who have the right of fostering the child after the mother; for more benefit in this regard, please refer to Fatwa 117955.
The maternal aunt is one of them. Therefore, if there is no person who comes in priority over you in fostering your nephew, then you have the right to foster him if there is no obstacle which prevents you from this right, like if you are married for instance. However, if it is someone else who has the right to foster him and renounces this right for you, then this is permissible.
To conclude, we advise you to agree with the father of the child and do what is beneficial for the latter. The fostering is for doing what is beneficial for the fostered child and repel the harm from him. If we presume that it is the father who has the right of fostering, it is not permissible for him to keep this child in an environment that may spoil him, whether his family or else. It is for this reason that some jurists conditioned when the fostering is transferred to the father or other male relatives, that he should have a female with him who would be suitable as a fosterer like a wife, or a female Mahram who is appropriate for fostering.
Allaah Knows best.