In the Original Series, Jeff and Sammy Jo married Fallon and Steven and were the parents of a new generation of Carringtons.
To date, only Sam has legally married Steven. Fallon and Jeff's marriage had been annulled.
According to Blake in the first season finale, he didn't learn about his half-sister existence before the day of Fallon and Jeff's annulled wedding.
In the original Blake and Alexis did have two other children : a son Adam, who was kidnapped as a baby and raised by another family, and a daughter Amanda, born after their split.
In this reboot only Adam is confirmed to have had existed. He was kidnapped as a baby and his father believes him to be dead, unlike his mother who lost all her money to search him. Fallon is the last member of the immediate family to learn about his existence.
Amanda is indirectly referenced in Trashy Little Tramp when Fallon asks Steven if he's been "Sleuthing for more siblings and found her a sister?"
In the original series, Fallon and Jeff were married and were also second cousins (their mothers were cousins).
In the reboot, their marriage was declared illegal before they learned that they are actually half-cousins (Fallon's father and Jeff's mother are half-siblings).