Mark 3:35

οὗτος ἀδελφός μου καὶ ἀδελφὴ καὶ μήτηρ ἐστίν.

"this one the brother of myself, and a sister and a mother is."

A mixed gender reference (brother, sister, and mother) with a masculine singular demonstrative pronoun?

This presents a challenge as there are multiple subjects but only one verb, "is" which must 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 adding words, without being crafty, ἐστίν can only fit as a feminine singular "she is" otherwise one is forced to read it as "he is a sister/mother" which is an absurd thing to say, no matter how you try to read it.

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 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 "to be" 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 whole verse. 

The gender disagreement of the "scholarly" interpretations can be seen even in the "most accurate" translations, with most removing the word οὗτος, or putting the masculine singular demonstrative pronoun out of view.

"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 cryptic sayings. This is especially the case when a saying is preceded by the word "behold!/pay close attention!"