From R Twersky’s “Dear Rabbi, Dear Doctor” p. 240, shortened
Depression in Jewish families is far more prevalent than realized. ..often kept secret [as well as other conditions],. choice is between shidduch with a family with a known case or the one that kept it secret [or other condition]. One father discussing this mentioned “while there are no cases in our family”. I had to restrain myself from saying “foolish man! I treated your wife before she married you!”
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many opt not to reveal problem, some may claim that a posek told them so. Poskim I consulted said it is unthinkable that anyone would give such a ruling… vilolatin of “lo sonu”, do not deceive, and probably of lifnei ever.
if condition is not likely to recur, then maybe no need to disclose. If likely to recur, I can not see how info may be withheld. I asked several poskim to issue a statement about it. They all said – no need, as no posek will ever advise withholding.
A marriage which begins with violation of trust is headed for a trouble. He brings a case when a wife dicsovered her husband’s pills, who said his parents told him not to disclose. As a result, marriage survived, but in-laws were cut off from the family. In another case of wife’s undisclosed depression, beis din justified a get.
In case of a family member with a disease, it is a question for geneticists, one told me that information should be made available but should not serve as an impediment. Everyone has something in their genetics.
I do not believe that the shadchan need to be told. But when it becomes a serious consideration, after several dates, the only thing to do is to reveal
As far as others, ask a [posek]. My understanding of Chafetz Chaim that if someone knows important information, he is required to tell the other party even if not asked