I have all the info on Nina. . . . She was my Dads aunt. . . . Remember her well.. . . . But I have no info on the O'Sullivan's other than she married Gerry .. . .I have him as being Gerry B O'Sullivan. . . . And that is all I have on Gerry. . . . I dont even know which county or Provence he was from but the settled Dublin area. . . .
But you have the son John, age 11, 1901 census, so ..so you can look for John's Birth, get mother's name, then look for first Marriage.
James says on Census he was
born Dublin Cityhttp://www.census.nationalarchives.ie/pages/1901/Dublin/Mountjoy/Upper_Cumberland_Street/1328529/Find First Marriage, then look maybe 5 years each way for any siblings Marrying.
James says he was 37 on 1901 Census so you MIGHT get his Birth Reg. Then check Births up to 1870 or after to look for siblings.
Marriage of JAMES OSULLIVAN and JANE KYLE was on 04 August 1895 so Jane wasn't John's Mother!!
All one can do is follow the clues.