Sunlover1 answered accurately. Would you trust your parents to just anyone?
Prior to the cross, the brothers of Jesus (same mother, different father) didn't believe in Christ (John 7:5). John did. The honor fell to him.
PS. John as cousin also had the right to care for Mary.

As you can see in Matthew 12, they and she were not even
"inside" where they should have been.
Not sure why they weren't focusing on His Father's business,
as He was.
Gill's Exposition of the Entire Bible
Then one said unto him,.... Either one of his auditors, or, as the Ethiopic version has it, one "of his disciples": the other evangelists intimate, that more than one acquainted him with it; which is easily reconciled: for, upon his mother and brethren calling to him, as Mark says they did; first one and then another, and more, might apprise him of it, and especially as he did not immediately go out unto them.
Behold, thy mother and thy brethren stand without, desiring to speak with thee: whether this message was carried at the request of the mother and brethren of Christ, and delivered in a simple manner, and with an honest intention; or whether it was officiously done, and with a design to interrupt him, and to try him, whether he would prefer his natural relations, and their society and conversation, to the spiritual work in which he was engaged, in doing good to the souls of men, is not certain; the latter seems probable, from the following words, and conduct of Christ. Some copies read, "desiring to see thee".