Please excuse me for answering in English but I am unable to type in Lashon ha'Kodesh.

You are asking on Tosfos from your understanding of the Gemara and from the Nimukei Yosef, but you must admit that the Gemara does not say differentiate between the Baal Davar and other K'rovim. In fact if anything, it compares them, as we shall see.

The Yosef Da'as explains that there are two ways of understanding the Gemara. The Nimukei Yosef actually agrees with the opinion of the Ra'avad, who explains that normally we say 'Eidus she'Batlah Miktzasah, Batlah Kulah', and that is the Din by the case cited by Tosfos, since the Eidus regarding his sister is Bateil.

It is only regarding himself that his testimony is not even called Eidus to begin with, and therefore allows the Eidus regarding P'loni to be Chal. As far as his wife is concerned, the Gemara initially thinks that she is like any other Karov, but concludes that 'Ishto ke'Gufo'.

From the Rambam however, it seems (says the Yosef Da'as) that he does not differentiate between the Ba'al Davar himself and other K'rovim (and it seems to me that neither does Rashi). And he adds that this is certainly the Shitah of Tosfos (with reference to the very Tosfos to which you are referring).

Indeed, the Gemara itself invalidates the Ba'al Davar purely because he is a Karov, in which case there is no difference between himself, his wife and any other Karov.

As for the Kashya why we do not say Eidus she'Batlah Miktzasah ... , see Yosef Da'as DH 've'Efshar Lefaresh'.

R. Eliezer Chrysler