It is a Mary allright, but a different one.
Not trying to gang up on anyone here, but if this Mary was a "different" one, then where is Jesus' mother even mentioned as being at the cross in this passage or at the tomb in the next chapter? This Mary is the same.
Of course she is a different Mary since in a passage about death of Jesus it is impossible to characterize His mother as the mother of everybody but Jesus. Evangelist Mark does not describe Mary the Mother of God at the foot of the Cross at all. Evangelist John does (John 19), and unlike St. Mark, he was there. John also mentions the other Mary, and gives us her full name:
there stood by the cross of Jesus, his mother, and his mother's sister, Mary of Cleophas, and Mary Magdalen. (John 19:25)
Note, once more, the use of "sister" to indicate a wider relationship than "daughter of the same parents", since obviously no one would give two daughters the same name.