Luke 2:34 - "And Simeon blessed them, and said unto Mary his mother, Behold, this child is set for the fall and rising again of many in Israel; and for a sign which shall be spoken against; (Yea, a sword shall pierce through thy own soul also,) that the thoughts of many hearts may be revealed."
The statement that a sword would pierce her own soul also is in parenthesis. The emphasis is on Jesus, the child.
Now, concerning the sword that would pierce Mary's heart also, consider Hebrews 4:12 - "For the word of God is quick, and powerful, and sharper than any two edged sword, piercing even to the dividing asunder of the soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart. Neither is there any creature that is not manifest in his sigh: but all things are naked and opened unto the eyes of him with whom we have to do. Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession."
The sword that pierced her own soul is the same sword that laid bare the thoughts and intents of many a heart. An example of this is the changing of water to wine. She said to Him, "They have no wine." His response was, "Woman, what have I to do with thee? mine hour is not yet come." Obviously He knew exactly what she wanted Him to do.