Railgun is level #5 esper, but her "sisters" only are level #2, #3 or #4, regardless they have exactly the same DNA. Why is this so?
Because Tree Diagram showed it was actually impossible to clone a Level 5 "Railgun" which was the original plan
Before mass production of the clones could begin, the simulation that the Tree Diagram produced showed that it was impossible to create a level 5 clone of Mikoto despite genetic manipulation and use of the Testament. It showed that the clones' specifications reached less than 1% of that of the real Mikoto's, which was equal to that of a level 2.
The original plan was to make "Railgun" clones for military purposes.
the Level 6 Shift program, the one to make Accelerator a Level 6, reused the previous work by calculating how many Level 5 "Railgun" it would take and then calculating the equivalent with the SISTERS
However, the Level 6 Shift program – an experiment designed to make Accelerator, the strongest level 5 in Academy City reach level 6 – reused the project. The project began after Tree Diagram calculated that Accelerator killing Mikoto 128 times in battle, would allow him to achieve level 6; however, since 128 level 5 Mikoto clones could not be prepared, it was calculated by Tree Diagram that Accelerator killing 20,000 clones at level 2-3 would achieve the same results.
i suspect that the Level 2-3 range is because the Misaka Network is similar to the Level Upper Network and Tree Diagram probably accounted for this (while Last Order is listed as a Level 3 while all the other clones are listed as Level 2-3, given she was made as a fail-safe to stop the SISTERS i don't think she was planned to fight as her ID is 20001 and outside Tree Diagrams 20000 calculation)
however from the beginning the second plan was designed to fail anyway because
Aleister Crowley purposely wanted it to fail so he could have the sister clones distributed all around the globe for the massive AIM-field, which was a crucial part to his plan to eliminate magic
but her "sisters" only are level #2, #3 or #4
Actually the only Level 4 SISTER is Misaka Worst who was shown to be difference in physical appearances (as noted by the linked Wikia, Misaka Worst is taller and have a larger chest compared to the SISTERS or Misaka). Misaka Worst was also created after Index accidentally destroyed Orihime 1 and Tree Diagarma and outside the Level 6 Shift Program