Moshe was making arrangements for his wedding. “I’d like you to sign as one of the witnesses on the kesubah,” he told Dovid, his best friend and long-time chavrusa, who gladly accepted.

At the wedding, Dovid said: “When I get married, I want you to sign my kesubah!”

A shidduch was suggested for Dovid — Moshe’s sister — and it was a perfect match!

At the vort, Dovid reminded Moshe of his earlier commitment. “I told you last year that I’d honor you to sign my kesubah,” he said.

“I can’t sign,” Moshe pointed out. “I’m disqualified as an eid. We’re no longer just close friends; we’re becoming relatives!”

“Too bad,” replied Dovid, with a grin.

Dovid’s wedding was celebrated with joy. On the way home, Moshe began thinking. “Dovid just became my brother-in-law,” he said to himself. “A brother-in-law is also a disqualified relative. Is my kesubah still valid? After all, one of the witnesses who signed just became disqualified.”

When sheva brachos were over, Moshe shared his concern with Dovid. “That’s an interesting question,” he replied. “It seems strange that your kesubah should become disqualified retroactively. Yet, if you were to separate, chas v’shalom, and your wife would want to actualize the kesubah, I don’t see how she could claim money from you based on my signature, since I’m now your relative!”

Moshe and Dovid decided to discuss the issue with Rabbi Dayan.

“Dovid signed on my kesubah a year ago and just became my brother-in-law,” Moshe said. “Is the kesubah still valid? What if it would need to be actualized?”

“This question touches on some fascinating points about witnesses,” replied Rabbi Dayan. “Although a brother-in-law is disqualified to testify or sign as a witness, the Gemara (B.B. 159a) teaches that the kesubah remains valid. This is because when witnesses sign on a document it is considered as if their testimony is accepted and verified by beis din at that point. This concept is known in Halachah as ‘Eidim hachasumim al hashtar — naasah k’mi shenechkera eidusan b’veis din’ (Kesubos 18b).

“When Dovid signed your kesubah last year, he was not a relative and was qualified to be a witness,” continued Rabbi Dayan. “Thus, his signed testimony remains valid even after becoming your brother-in-law. However, should a need arise in the future to validate his signature in order to actualize the kesubah, Dovid could no longer testify to validate it, since he is now your brother-in-law” (C.M. 46:35; Sma 46:86).

“What could be done, then?” asked Moshe.

“Other people, who recognize his signature, would have to validate it,” Rabbi Dayan replied. “Alternatively, beis din could compare it with other validated signatures of his” (C.M. 46:7).

“Would this apply to other disqualified witnesses?” he asked. “For example, what if one of the witnesses later became a thief?”

“The same would apply, provided we knew of the existence of the document before the person became a thief. Otherwise, we are concerned that the thief signed a false document now, but predated it before he became disqualified” (C.M. 46:34).

“Does the requirement to know about the document apply also to a relative?”

“No. While the Torah disqualifies a relative as a witness, he is not necessarily suspected of being dishonest,” replied Rabbi Dayan. “For example, he cannot testify even to the detriment of his relative. Even Moshe and Aharon are disqualified as witnesses for each other, despite their impeccable integrity. The Rambam calls the disqualification of relatives a gezeiras hakasuv (Scriptural decree). Once their signature is validated, we do not suspect that they signed after becoming relatives and predated the document. Thus, their signed testimony remains valid” (Hil. Eidus 13:15; Shach 46:92).