In mitosis, the sister chromatids align in the "center" of the cell as the centrosomes are at the poles of the cell. They do this, moving on to the next phase Anaphase - where they will eventually split. Interphase isn't part of mitosis - it describes the period in which the cell is just growing and copying DNA. If you textbook says it is, then thats ok, sometimes its included as part of it (even though technically its not).
