Real Bible Translation

The phrasing οὗτος...ἀδελφὴ ἐστίν is grammatically anomalous in Classical and Koine Greek due to the discordance in gender agreement between the masculine singular demonstrative pronoun οὗτος and the feminine singular predicate noun ἀδελφὴ. Standard Greek grammar generally requires agreement or at least logical alignment in gender between the subject and predicate nominative, especially in copulative constructions. The verb ἐστίν is singular but does not mark gender explicitly; however, the predicate noun’s gender typically corresponds to the subject’s. Thus, a masculine pronoun governing a feminine predicate noun without contextual justification is considered incorrect or, at minimum, highly unusual. Acceptable alternatives would be οὗτος ἀδελφός ἐστίν (“this one is a brother”) for masculine agreement or a feminine demonstrative such as αὕτη ἀδελφή ἐστίν for feminine agreement.

The masculine generic use often appears in plural or collective contexts (e.g., οἱ γονεῖς “parents” includes both genders but uses masculine plural form).
It can also appear in poetic or rhetorical contexts where meaning overrides strict grammatical gender, often in attributive adjectives or periphrastic expressions.
However, this does not extend naturally to singular feminine nouns serving as predicate nominatives with a masculine singular subject and demonstrative.
Thus, a phrase like οὗτος ἀδελφὴ ἐστίν remains grammatically irregular and unexplained by the general flexibility described in Smyth §§1013–1015.

This presents a challenge as there are multiple subjects but only one verb, "is" which must also agree in number. There is no specific gender form to the verb. The verb ἐστίν "he/she/it is" is not plural "they are." The masculine demonstrative pronoun οὗτος "this one" specifically refers to the brother. Further, there should be agreement with the masculine demonstrative pronoun. A neuter singular could have been used (τοῦτο ἀδελφός μου καὶ ἀδελφὴ καὶ μήτηρ ἐστίν) or better, a plural, "These ones are" but this is not the case. Why?

To accomodate all the syntax present in this verse without breaking syntactical rules, without changing or adding words, without calling a brother 'a mother', ἐστίν can only fit as a feminine singular "she is" otherwise one is forced to read it as "this one is a sister/mother" which is an absurd thing to say (unless you live in the 20th century), which is why traditional translations have tampered with the Greek.

In addition, the possessive μου in this clause is directly applied to "brother" and not "sister" or "mother." In Ancient Greek, οὗτος ἀδελφός μου is a grammatically correct way to say "this is my brother", as the verb "to be" (εἰμί) is often omitted when it is implied by the context. This is common in Greek, especially in simple sentences where the meaning is clear. Relevant biblical examples can be seen in John 21:14:

"Τοῦτο ἤδη τρίτον..."

"This is already the third..."

Luke 4:36:

"Τίς ὁ λόγος οὗτος?"

"Who/what is this Word?"

Luke 8:35:

"Ποῦ ἡ πίστις ὑμῶν"

"Where is the faith/trust of yourselves?"

The verb "is" is implied. Furthermore, the masculine singular demonstrative pronoun οὗτός is almost always followed immediately by the verb "to be" in the NT (οὗτός ἐστιν). Here in Mark 3:35 it is missing, and the verb "to be" is found much later at the end of the verse.

The gender disagreement of the "scholarly" interpretations can be seen even in the "most accurate" translations:

"he is my brother and sister and mother." (ESV)

"this is My brother, and sister, and mother." (NASB)

"he is My brother and sister and mother" (BLB)

"is my brother and sister and mother." (NIV)

"the same is my brother, and my sister, and mother." (KJV)

"he is my brother, and my sister, and mother." (YLT)

Grammatical difficulties like this exist, not as a matter of poor writing, scribal error, or otherwise, but as a matter of careful intention, even enigmatic sayings. This is especially the case when a saying is preceded by the word "behold!/pay close attention!"