None of the Protestant Bibles contain the word "mother" in the verse.
Douay-Rheims Bible
And when his friends had heard of it, they went out to lay hold on him. For they said: He is become mad.
New International Version (©1984)
When his family heard about this, they went to take charge of him, for they said, "He is out of his mind."
New American Standard Bible (©1995)
When His own people heard of this, they went out to take custody of Him; for they were saying, "He has lost His senses."
International Standard Version (©2008)
When his family heard about it, they went to restrain him, because they kept saying, "He's out of his mind!"
GOD'S WORD® Translation (©1995)
When his family heard about it, they went to get him. They said, "He's out of his mind!"
King James Bible
And when his friends heard of it, they went out to lay hold on him: for they said, He is beside himself.
American King James Version
And when his friends heard of it, they went out to lay hold on him: for they said, He is beside himself.
American Standard Version
And when his friends heard it, they went out to lay hold on him: for they said, He is beside himself.
Bible in Basic English
And when his friends had news of it, they went out to get him, saying, He is off his head.
Darby Bible Translation
And his relatives having heard of it went out to lay hold on him, for they said, He is out of his mind.
English Revised Version
And when his friends heard it, they went out to lay hold on him: for they said, He is beside himself.
Webster's Bible Translation
And when his friends heard of it, they went out to lay hold on him: for they said, He is beside himself.
Weymouth New Testament
Hearing of this, His relatives came to seize Him by force, for they said, "He is out of his mind."
World English Bible
When his friends heard it, they went out to seize him: for they said, "He is insane."
Young's Literal Translation
and his friends having heard, went forth to lay hold on him, for they said that he was beside himself,
Perhaps you were reading the commentary part of the Bible. For instance, Wesley's bible notes concerning this verse is:
Mr 3:21 His relations - His mother and his brethren; Mr 3:31. But it was some time before they could come near him.