And I don't think St Paul's intention in this passage is to teach us that the Theotokos sinned, otherwise he would have at least mentioned Her somewhere.
St. Paul wrote what he was inspired to write, nothing more nothing less. Perhaps the reason he didn't bother to mention her is because he didn't consider her as any different than the rest of the apostles with regards to her sin status?
A biblical passage that I encountered the other day really jumped out at me with regards to this issue. Mark 3:20-30:
"
20 And the multitude cometh together again, so that they could not so much as eat bread.
21 And when his
friends heard of it, they went out to lay hold on him: for they said, He is
beside himself.
22 And the scribes which came down from Jerusalem said, He hath Beelzebub, and by the prince of the devils casteth he out devils.
23 And he called them unto him, and said unto them in parables, How can Satan cast out Satan?
24 And if a kingdom be divided against itself, that kingdom cannot stand.
25 And if a house be divided against itself, that house cannot stand.
26 And if Satan rise up against himself, and be divided, he cannot stand, but hath an end.
27 No man can enter into a strong man's house, and spoil his goods, except he will first bind the strong man; and then he will spoil his house.
28 Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme:
29 But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation.
30 Because they said, He hath an unclean spirit.
31 There came then his brethren and his mother, and, standing without, sent unto him, calling him. 32 And the multitude sat about him, and they said unto him, Behold, thy mother and thy brethren without seek for thee.
33 And he answered them, saying, Who is my mother, or my brethren?
34 And he looked round about on them which sat about him, and said, Behold my mother and my brethren!
35 For whosoever shall do the will of God, the same is my brother, and my sister, and mother."
In vs. 21, friends here literally means "his own" or "his family". Most translations render this as "family". Also, from vs. 31 it is clear that his family were the ones being alluded to prior. Also in vs. 21, Being "beside oneself" in this sense indicates being "out of one's mind".
From this passage, is it not painfully obvious that his mother was indeed numbered among the rest of his family members who went to retrieve Him because they thought He was beside himself? Even his apostles denied him or doubted Him at one point; would it be surprising to think that his own family may have done the same?
Mark 6:4 gives us more information that would lead us to believe that they did.
4Jesus said to them, “Only in his hometown, among his relatives and in his own house is a prophet without honor.”
5He could not do any miracles there, except lay his hands on a few sick people and heal them.
6And he was amazed at their lack of faith.