Yes, if it is crucial information and would affect the success of the marriage, and you are asked for your input, you must be honest.
This situation is among the cases where gheebah (backbiting) is permissible, or maybe could even be obligatory.
Of course, you must be careful that it does not somehow backlash onto you as well.
You should perform the obligatory Daily Prayers in a place where your non Muslim family should not see you, either in a separate room at home or outside your home.
Islam does organize the family life and the relationship between husband and wife but there no right for the husband to stop his wife from meeting her family or usual friends who don't harm her faith and behaviour. Working of the wife depends on its nature, so if her work does not prevent her husband from his rights as a husband, then it is allowed for her to work, and vice Versa.
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