What is the ruling on a wife not wearing the hijab in her non-Muslim family's home and concealing her Islam from them, and is the husband considered a "dayyuth" (one who tolerates indecency concerning his womenfolk) and complicit in sin in this case, and what should be done?

Your wife has no excuse for uncovering her head and wearing pants, unless she fears harm from her family. She is not among the "oppressed" mentioned in the Quran, because she does not fear for her life from them, but rather fears their estrangement. It is her duty to inform her family of her conversion to Islam, to continue to maintain ties with them and treat them kindly, and to strive to invite them to Islam.