Regarding Yitzchok-look in the Rashi in Parshas Vayeishev perek lamed zayin, pasuk lamed gimmel. Rashi quotes Bereishis Raba that Yitzchok knew, but did not reveal this information because he said “if HKB”H doesn’t want to reveal it then surely I can’t”. There is also and explanation that the brothers put a curse on whoever would reveal such information to their father and made Hashem part of that ban and curse. Binyomin however I believe was not part of that ban because he was not present.
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