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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The girl breastfeeding from the wife of your husband's nephew at this age does not make her a Mahram (non-marriageable) to your husband. In fact, even if she breastfeeds from you (his wife), then this breastfeeding would still have no effect. This is so because the Islamic period of breastfeeding is over according to the preponderant opinion of the scholars.
This is the view of the majority of the jurists . For more benefit on the duration of breastfeeding in Islam, please refer to Fatwa 91172 and 86750.
On the other hand, being kind to this girl, bringing her up and taking care of her is an act by which one gets closer to Allaah The Almighty. However, it is not permissible for you to attribute her to your husband as she is not really his daughter and there is no (blood) relation between them; rather, she is non-Mahram (marriageable) to him. Thus, if she reaches puberty, it is not lawful for him to stay in seclusion with her or do other matters with her that are forbidden to do with a non-Mahram woman.
For more benefit on the ruling of adoption, please refer to Fataawa 84422, 88415, 86418, 150319 and 119119.
Allaah Knows best.