And how did the North supposedly own this property that was part of another sovereign State?
By act of the South Carolina legislature:
COMMITTEE ON FEDERAL RELATIONS
In the House of Representatives, December 31st, 1836
The Committee on Federal relations, to which was referred the Governor’s message, relating to the site of Fort Sumter, in the harbour of Charleston, and the report of the Committee on Federal Relations from the Senate on the same subject, beg leave to Report by Resolution:
Resolved, That this state do cede to the United States, all the right, title and claim of South Carolina to the site of Fort Sumter and the requisite quantity of adjacent territory, Provided, That all processes, civil and criminal issued under the authority of this State, or any officer thereof, shall and may be served and executed upon the same, and any person there being who may be implicated by law; and that the said land, site and structures enumerated, shall be forever exempt from liability to pay any tax to this state.
Also resolved: That the State shall extinguish the claim, if any valid claim there be, of any individuals under the authority of this State, to the land hereby ceded.
Also resolved: That the Attorney-General be instructed to investigate the claims of Wm. Laval and others to the site of Fort Sumter, and adjacent land contiguous thereto; and if he shall be of the opinion that these parties have a legal title to the said land, that Generals Hamilton and Hayne and James L. Pringle, Thomas Bennett and Ker. Boyce, Esquires, be appointed Commissioners on behalf of the State, to appraise the value thereof. If the Attorney-General should be of the opinion that the said title is not legal and valid, that he proceed by seire facius of other proper legal proceedings to have the same avoided; and that the Attorney-General and the said Commissioners report to the Legislature at its next session.
Resolved, That this House to agree. Ordered that it be sent to the Senate for concurrence. By order of the House: T. W. GLOVER, C. H. R.
IN SENATE, December 21st, 1836
Resolved, that the Senate do concur. Ordered that it be returned to the House of Representatives, By order: JACOB WARLY, C. S.
Sumter was not the joint property of the U.S. and South Carolina. Such a relationship would be unconstitutional since Article I, Section 8 says that Congress will exercise sole legislative authority over forts. Even if the South Carolina act of secession was legal, that did not automatically transfer control to them of property that belonged to the federal government. Sumter was the property of the federal government and South Carolina had no claim to it.
"And how did the North supposedly own this property that was part of another sovereign State?"
"Own" might not be the correct word, but Federal money was used to purchase (from Spain and France) the land which comprised many of the Confederate States that were not part of the original 13 colonies. Federal resources (blood and treasure) were used before the Civil War to defeat lingering Mexican claims to Texas. Federal resources were also used to purchase land in the South from Indian tribes and to "relocate" them.