Nimtza Echad Mehem Karov O' Pasul

There is a halacha in Hilchos Eidus that if even one of the eidim in a set of witnesses is found to be a karov (a relative) or a passul, the entire set of witnesses is thrown out. Based on this halacha there is a famous question dealt with by the rishonim as to how anyone can ever get married. Assuming that at the wedding there is at least one relative who witnesses the maaseh kiddushin, shouldn't we say that nimtza echad mehem karov o' passul eidusan b'tailah. Since one of the witnesses to the marriage is a relative, all the witnesses should be thrown out, and the kiddushin should be nullified.

This issue is addressed by Tosafos in Makkos on daf 6a. Tosafos answers that shaas reiyah alone is not enough to be mitztaref eidim for this halacha. In other words, the halacha of nimtza echad does not kick in at the time the event is witnessed. Rather, if a group of eidim are involved in hagadas eidus (i.e. they are testifying to something in court), only then would the halacha apply.
The Ritva brings a Ramban in Kiddushin with another answer, that the husband can be meyached eidim. If the husband says that these two people are my eidim for the kiddushin and no one else, so everyone else in the crowd is automatically excluded from the eidus, and the kiddushin is valid. Based on this shitah some are noheg in weddings to announce "So and so are eidei kiddushin... to the exclusion of all others".

One question on the Ramban is, what kind of crazy idea is this to be meyached eidim? If someone is about to commit retzicha, can he exclude people who are present from being eidim?

I heard over in the name of R' Soloveitchik that this is one of the reasons to say that when it comes to eidus l'kiyum hadavar, the eidus is actually a part of the maaseh habaal. There are two distinct categories of eidus. One is eidus l'birur hadavar. These are eidim that determine what happened. For example, eidim witness a maaseh retzicha and clarify what precisely took place. Obviously, there is no yichud eidim in this scenario. However, by marriage (and divorce) the eidim are l'kiyum hadavar. They are a part of the maaseh habaal. By being present at the time of the kiddushin they allow the daas of the baal to create the challos kiddushin. They aren't determining what happened, they are actually enabling it to happen. Since eidim l'kiyum hadavar are a part of the maaseh kiddushin, we can imagine a concept of yichud eidim. (For more on this issue of eidus l'kiyum hadavar see Rav Chaim in Yibbum V' Chalitza which we mentioned in this post.)

The Ktzos in Siman 36:6 also addresses this issue of how our weddings work. He gives an answer that is similar to the Ramban's.