Probably in two years, Consuela "Connie" Rubirosa will be joining other 'Law & Order' alumni in the Television Crossover Hall Of Fame. She was a regular on the flagship as an ADA before moving to Los Angeles to work there in 'Law & Order: LA'. And she was also a guest character on 'Law & Order: Special Victims Unit'.
In the "Captive" episode of 'Law & Order', Rubirosa told her boss, Jack McCoy, about her sister, who had been in an abusive relationship.
I think this sister was Connie Rubirosa's twin, Jo Martinez, who is currently working in TV-NYC as a police detective, as seen in 'Forever'.
The difference in last names is easy to splain away: we know Detective Martinez is a widow. But she could also have taken the last name of their step-father, who married their mother after she divorced their father. Connie chose to keep her father's name. (But I think the last name of Jo's husband makes an easier splainin.)
The abusive relationship was before Jo married her husband. She talks too fondly of him with her "partner in crime", Dr. Henry Morgan.
Should it ever come up that a sister of Detective Martinez appears in an episode of 'Forever', then she would be another sibling never mentioned by Connie. (She did mention a brother once in a 'Law & Order' episode.) And if Martinez claims that she has no sister at all, well.... There must have been bad blood between Connie and Jo at some point.