Shulchan Aruch (E.H. 2:5): If a family of Cohanim has a definite challel amongst them then every woman of that family is prohibited until she is checked and if she marries she must be divorced. The same applies if there is a family that a sofek mamer or definite mamzer.
Rema: However all of this applies only if it is definitely known to be true. However a family that has a person who is invalid for marriage – but it is not public knowledge- since he has been successfully mixed in then let him remained so. Someone who knows about this invalid person – is not allowed to reveal the information but the family members are to be left with their presumption of validity (chezkas kashrus). That is because all such families which have an unknown invalid person amongst them will be declared as fully kosher in the Messianic Era. Nevertheless is appropriate to reveal this information to those who can keep a secret (This is implied by the Ran). However this ignoring of the information is only when the invalid member is successfully mixed in and concealed from public knowledge. But as long as he is not mixed in then we reveal the invalid members and publicly announce it so that unblemished people will avoid marriage with them. See Choshen Mishpat 25 as to who is believed to testify about a families pedigree.