The 13 cents of land given to X first needs to demarcated and identified and a boundary needs to be made covering this land
Only then would it be said that the above land is duly divided or partitioned by metes and bounds
However there is no stopping against X to sell the 2nd floor allotted to her and also her rights in the 13cents of land as and when identified in future
X has in praesenti rights over the undivided land ie 13 cents and would obviously have future rights over the divided land, as and when divided
So X can sell second floor alongwith in praesenti rights in undivided share of land and alongwith future rights to the divided land corresponding to the area of undivided land presently held
In essence the buyer of X will have a right in future to obtain partition of land from X's sister