Isilmo was the younger brother of Queen Tar-Telperiën. He had a son, Tar-Minastir. Despite that, Tar-Telperiën outlived Isilmo; she refused to wed and had no children. Thus, Tar-Telperiën was succeeded by Isilmo's son, her nephew, who became Tar-Minastir.
Some fans have speculated that Isilmo was the man who became the Lord of the Nazgûl:
Isilmo lived during in the correct timeframe. He was the only Númenórean prince to be passed over for the sceptre of Númenór in favor of a sister. This had not been the prior Númenórean practice, and all that was said about the circumstances was that "it was made a law of the royal house." This may have been a motivation for Isilmo to stray. In The Lord of the Rings, the Lord of the Nazgûl wore a kingly crown, which may have symbolized Isilmo's unfufilled claim to kingship of the Númenóreans instead of merely the obvious kingship of Angmar.
Like Isildur (the founder of Minas Ithil the "tower of the moon"), Isilmo's name has the Sindarin word for "moon" as its root; and the livery of Minas Morgul was that of a "moon disfigured with a ghastly face of death" which may have signified Isilmo as the corrupted Lord of the Nazgûl.
There is insufficient evidence in Tolkien's works to say authoritatively either way, and other fans have vehemently disagreed with the above speculation.