Gemara

(Mishnah - R. Akiva): If one of three witnesses was found to be a relative or Pasul, the testimony is invalid.

R. Yosi says, this applies only to capital cases. For monetary cases, the testimony of the others is valid.

Rebbi says, capital and monetary cases are the same. This is only if they warned (the transgressor). If not, if two brothers witnessed a murder, what can they do (to avoid disqualifying all the witnesses)?!

(Rava): We ask them if they came just to see, or in order to testify. If they came just to see, the testimony of the others is valid.

(Rav Yehudah): The Halachah follows R. Yosi.

(Rav Nachman): The Halachah follows Rebbi.

Bava Basra 113b (Rav Yehudah): If three men came to visit a sick man (and he commanded who should inherit his property), they may write down his command, or execute it, whichever they want. If two came, they write down his command, but may not execute it.

(Rav Chisda): This applies only during the day. At night, even three can only write, and they cannot execute it (even the next day).

This is because when they heard the command, they were able to be only witnesses, not judges, and a witness cannot become a judge.

Sanhedrin 28a - Question: What is the source that two relatives cannot testify together (about someone else)?

Answer #1 (Rami bar Chama): Reasoning teaches this. Witnesses are not made Zomemim unless both are Huzmu. If relatives could testify about someone else, they could be killed due to each other's testimony!

Objection (Rava): If relatives testified together and were Huzmu, they are killed due to the Hazamah, not due to each other's testimony!

Answer #2 (Rava): Rather, the Torah could have said 'Ben Al Avos' or 'Hem Al Avos.' Instead, it says "u'Vanim" to teach that Banim (i.e. relatives) cannot testify with each other.

Rishonim

The Rif (Bava Basra 52b) brings the Gemara in Bava Basra.

Rif (Makos 2b): In capital cases, testimony of the others is valid when the Pesulim did not warn, for they came only to see. There is no warning in monetary cases. If the other witness did not know that the lender or borrower has a relative here, the testimony of the others is valid.

Rambam (Hilchos Edus 5:8): In capital cases, a witness who testified may not give a reason to convict or acquit the Nidon. In monetary cases, a witness may give reasons to convict or acquit the Nidon, but he may not be counted among the judges.

Rambam (Hilchos Sanhedrin 3:6): If two came to visit a sick man and he commanded in front of them, they write down his command, but may not execute it. If three came, they may write down his command, or execute it, whichever they want. This applies only during the day. At night, even three can only write, and they cannot execute.

Kesef Mishneh: The Rashbam says that this is only when they entered to visit, but if they intended to testify, they are witnesses, and they cannot become judges. He brings a proof from Makos 6a. We ask witnesses if they came just to see (the event), or in order to testify (about it later). If they came to testify, and one of them is relative or Pasul, all the testimony is Batel. The Rambam explains like this; Tosfos explains differently.

Rosh (Bava Basra 8:2): The Rashbam says that if people were asked to be witnesses, they cannot become judges. This is wrong. The Yerushalmi says that witnesses may not testify in front of judges who are their relatives, and all the more so witnesses may not be judges, because it is Iy Efshar Lehazimah!

Poskim

Shulchan Aruch (CM 7:5): A witness who testified cannot become a judge (about this case). If one of the judges testified in front of his collegues about what he saw, he cannot join with them to judge it.

Chidushei ha'Ran (Bava Basra 113b DH Nimtza'u, brought in Beis Yosef DH v'Zeh): The Rashbam says that one who saw during the day with intent to testify is a witness, and cannot become a judge. If he saw at night, he could become a judge if others will testify. This is unreasonable. Why should he become a witness from when he intended to do so? If so, every Kesuvah and Kidushin is invalid. It is normal to be Mekadesh in front of relatives and strangers, without concern lest the relatives intended to testify. It says in Makos 'did you come to see, or in order to testify?' This does not refer to intent at the time of the event. Why would we assume that they came? Perhaps they were already there! Rather, we ask why they came to Beis Din. If the Pesulim came to testify, they disqualify the Kesherim. The Ramban and Rashba say that we ask them after they testified. If they did not mean to testify, they do not disqualify the Kesherim. However, it seems that if one was summonsed to be a witness, he may not become a judge, and if he is Pasul he disqualifies all the witnesses. Some say that being summonsed to be a witness is like seeing the event at night. He may judge based on others' testimony, but not based on his own. Others say that this is only when he saw at night; one who does not need another's testimony is not disqualified.If he was summonsed to be a witness, immediately he is a witness and he may never become a judge. One should rule like this.

Beis Yosef (ibid.): The Rambam connotes that a witness is disqualified from being a judge only if he testified.

Taz (DH Tu): The Beis Yosef connotes that in any case, if he did not testify, he can become a judge, even if he was asked to testify. However, this cannot be, for he wrote oppositely in the Kesef Mishneh (Sanhedrin 3:6).

Shach (4): Intent to testify makes one a witness regarding if one of the witnesses was found to be a relative or Pasul. Even though the Ran equates this to witnesses becoming judges, we follow Tosfos and the Rosh, who distinguish. Witnesses cannot become judges due to "the two men will stand in front of Hash-m", or because it is Iy Efshar Lehazimah. These apply only to a witness who testified.

Shulchan Aruch (ibid): If he did not testify, e.g. three judges saw an event, even if they intended to be witnesses, if they saw during the day they become judges. If they saw at night, they cannot judge by themselves; others must testify in front of them. If they were asked to be witnesses, they cannot judge even if others testify in front of them. Some say that even in this case, they can judge based on others' testimony.

Gra (23): The first opinion is the Rashbam. The Halachah follows Rav Nachman, who rules like Rebbi. The Rashbam holds that this is even if he was not asked to be a witness, but he saw with intent to testify later. Tosfos and other Poskim disagree, for if so, R. Yosi holds that a witness can never become a judge, but the Gemara in Rosh Hashanah held that obviously, seeing is no worse than hearing! Also, Rav Yehudah rules like R. Yosi, and he holds that three who came to visit a sick man may execute his command! Tosfos explains that it does not depend only on seeing the event, rather, also on coming to Beis Din to testify. The latter opinion is like Tosfos. The Ran holds like the Rashba regarding one summonsed to be a witness, and like Tosfos when he merely intended to be a witness.