if the matter is as you described, all the deceased person's debts, if any, should be paid, then any legal will, if any, must be implemented. The rest of his money should be distributed to his three children on the basis of Ta`sib (not having a specific allotted share, rather they get what is left after the allotted shares have been distributed), and his full sister shall be prevented from inheriting due to the presence of the deceased's children.
your bequest of the land to your nephew is not binding until the death of the testator and as
the person, who shall receive it, has passed away, the will is rendered invalid and, therefore, you are free to do with it whatever you like. (Part No. 16; Page No. 337) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.