If you knew which port she came into the US at, you could search the immigration records for people with the same surname. If there were others with Serafina's surname that immigrated the same time as she did, they would likely have been family members.
I am assuming you have already tried to obtain a death certificate for Serafina without success, which usually lists the parents of the deceased. Have you tried looking up the obituary for Serafina? It might list surviving relatives and their locations.