Should the sister be informed of the truth about her parents, taking into consideration her attachment to her uncle's family and his wife's refusal to let her go?

Allah has forbidden adoption, which is calling someone by a father other than their own, based on the Almighty's saying: ﴿Call them by their fathers' names; it is more just in the sight of Allah.﴾ So, if the sister is called by a father other than her own, this is not permissible. You must call her by her father's name and inform her of the truth about her lineage, especially since she has reached or is close to the age of puberty, so that she treats her adult male cousins as strangers to her.