When a person learns about an engagement for marriage within their family, is it their duty to inform their relatives if they have crucial information about the character of one of the parties or their family that might affect the marriage?
Bismihi ta'ala
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.
And Allah knows best