Hi Pearl,
Some guess work here.
Sebastiano was born on June 4, 1883, which makes him about 18 when he married. it is likely that he would have married a woman younger than he (or the same age).
Do you know if Rosa remarried after Sebastiano died. He died on March 20, 1935 as a result of infection from an injury suffered to his right arm in a fall on an icy street on Jan 21 1935. Remarriage would account for why I can't find her.
You might check the records at Calvary Cemetery in St. Louis, where Sebastiano is buried.
Sebastiano's mother was probably born in the 1860's.
Like I said, except for Sebastiano's birth & death, the rest is guestimates.
Angelina