Jerusalem Talmud Rosh Hashanah 1:6

משנה: אָב וּבְנוֹ שֶׁרָאוּ אֶת הַחֹדֶשׁ יֵלֵכוּ. לֹא שֶׁמִּצְטָרְפִין זֶה עִם זֶה אֶלָּא שֶׁמָּא יִיפָּסֵל אֶחָד מֵהֶן יִצְטָרֵף הַשֵּׁנִי עִם אַחֵר. רִבִּי שִׁמְעוֹן אוֹמֵר אָב וּבְנוֹ וְכָל־הַקְּרוֹבִין כְּשֵׁירִין לְעֵדוּת הַחֹדֶשׁ. אָמַר רִבִּי יוֹסֵי מַעֲשֶׂה בְטוֹבִיָּה הָרוֹפֵא שֶׁרָאָה אֶת הַחֹדֶשׁ בִּירוּשָׁלִַם הוּא וּבְנוֹ וְעַבְדּוֹ הַמְשׁוּחְרָר וְקִיבְּלוּ הַכֹּהֲנִים אוֹתוֹ וְאֶת בְּנוֹ וּפָֽסְלוּ אֶת עַבְדּוֹ. וּכְשֶׁבָּאוּ לְבֵית דִּין קִיבְּלוּ אוֹתוֹ וְאֶת עַבְדּוֹ וּפָֽסְלוּ אֶת בְּנוֹ׃ MISHNAH: A father and son who saw the new moon should go; not that they could combine one with the other²⁹²Since in formal testimonies close relatives are not permitted; these include close relatives to one of the parties, or one of the judges, or another witness. but that if one of them should be found disqualified the second one could team up with another person. Rebbi Simeon says, a father and his son and all relatives are qualified for the testimony of the new moon.
Rebbi Yose said, it happened that that Tobias the healer saw the new moon in Jerusalem, he and his son and his freed slave. The Cohanim²⁹⁴To determine that the sacrifices of the New Moon were due on this day. accepted him and his son and disqualified his slave; but when he came to the Court²⁹⁵To determine the date of the holidays. they accepted him and his slave but disqualified his son.

הלכה: טַעֲמָא דְרִבִּי שִׁמְעוֹן כַּתְּחִילָּתָהּ. וַיֹּ֤אמֶר יְי אֶל־מֹשֶׁ֣ה וְאֶֽל־אַֽהֲרֹ֔ן הַחוֹדֶשׁ הַזֶּ֛ה לָכֶם֭ רֹ֣אשׁ חֳדָשִׁ֑ים וגו׳. HALAKHAH: Rebbi Simeon’s reason: As at its start. The Eternal said to Moses and Aaron … This month be for you the first of months²⁹³Since the determination of the first New Moon was to be made by two brothers, one cannot be more restrictive in all later cases. Babli 22a..

קִיבְּלוּ הַכֹּהֲנִים אוֹתוֹ וְאֶת בְּנוֹ וּפָֽסְלוּ אֶת עַבְדּוֹ. מִשֵּׁם פְּסוּל. וּכְשֶׁבָּאוּ לְבֵית דִּין קִיבְּלוּ אוֹתוֹ [וְאֶת] עַבְדּוֹ וּפָֽסְלוּ אֶת בְּנוֹ. מִשֵּׁם קָרוֹב. “The Cohanim accepted him and his son and disqualified his slave,” because of disqualification²⁹⁶These are disqualified by rabbinic standards, Sanhedrin 3:6" href="/Mishnah_Sanhedrin.3.6">Mishnah Sanhedrin3:6, Notes 44–50. By biblical standards, only close relatives and criminals are disqualified., “but when he came to the Court they accepted him and his slave but disqualified his son,” because of relative²⁹⁷In criminal cases and civil cases decided by judicial ruling, not arbitration..