The gemarrah (Daf 159a), says even Moshe Rabeinu and Aharon Hakohen were unable to testify on their son-in-laws signature, even though they were not relatives at the time the SHTAR was signed and became son-in-laws afterwards.
It would seem that Moshe and Aharon's testimony should be believed even after they become related because of an UHMDENAH that nothing changed as far as the validity of the signature is concerned because it was signed before they were related. i.e it would have been accepted then, so too it should now.
(Maybe Reb Elchonon speaks about it.)
Pinny Stein, Montreal
The Edus of Kiyum ha'Shtar is not given at the time of the signing of the Shtar; rather, it is given at the time of the Kiyum. At the time of the Kiyum, they were relatives and were Pasul l'Edus. Even if there was an Umdena, the fact that Kerovim are Pasul is a Gezeras Melech, and because we are afraid that Moshe and Aharon are lying.
D. Zupnik