Why is Senator Grace Poe rumored to be the the daughter of Rosemarie Sonora?

Most probably NO.

Based on available resources, Grace Poe was born on September 3, 1968 in Iloilo City. Based on the list of Films made by Rosemarie Sonora, she could not possibly been pregnant during this period and went unnoticed by the viewing public.

1987 Hari sa hari, lahi sa lahi
... aka King and Emperor (International: English title)
1984  Campus Beat
1976  Ihalik mo ako sa diyos
... aka Kiss Me to God (literal English title)
1975 Mag-ingat kapag biyuda ang umibig
... aka Beware: When a Widow Falls in Love (Philippines: English title)
1972  Just Married, Do Not Disturb  (as Rosemarie)
1971 Cadena de amor (as Rosemarie) .... Sonya
1971 Life Everlasting (as Rosemarie)
1970 The Young at Heart (as Rosemarie)
1970 First Kiss (as Rosemarie)
1970 Gintong alaala (as Rosemarie)
1970 Twinkle, Twinkle, Little Star (as Rosemarie)
1969  9 Teeners (as Rosemarie)

1968 Bahay kubo, kahit munti (as Rosemarie)
1968 Baluga
1968 Ikaw ay akin, ako ay sa iyo (as Rosemarie)
1968 Juanita Banana (as Rosemarie) .... Juanita
1968 Magic Guitar (as Rosemarie)
1968 Order ni Osang (as Rosemarie)
1968 Pitong krus ng isang ina (as Rosemarie)
1968 Sandwich Shindig (as Rosemarie)
1968 Sayonara My Darling (as Rosemarie)
1967 All Over the World  (as Rosemarie)
1967 Bus Stop (as Rosemarie)
1966 Alis d'yan huwag mo akong ligawan (as Rosemarie)
1966 Ay ay naku Neneng (as Rosemarie)
1966 Jamboree '66
1966 Maraming kulay ng pag-ibig (as Rosemarie)
1965 Alaala ng lumipas (as Rosemarie)
1965 Magic bilao (as Rosemarie)
1965 Papa um mamaw (as Rosemarie)
1965 Rosalie (as Rosemarie)
1964 The Dolly Sisters (as Rosemarie)
1964 Magkakapatid na waray (as Rosemarie)
1964 Batang artista, Mga (as Rosemarie)
1964 Batang bakasyonista, Mga (as Rosemarie)
1964 Batang iskwater, Mga (as Rosemarie)
... aka Batang Squatter (Philippines: English title)
1964 Bata ng lagim, Mga (as Rosemarie)
1964 Batang milyonaryo, Mga (as Rosemarie)
1964 Jamboree (as Rosemarie)
1964 Show of Shows (as Rosemarie)
1964 Umibig ay di biro (as Rosemarie)
1963 Kwela sa eskwela, Mga (as Rosemarie)
1963 Class reunion, Ang (as Rosemarie)
1963 Dance-O-Rama (as Rosemarie)
1958 Ulilang angel (as Rosemarie)

Rosemarie Sonora was born on April 14, 1948 in Bacolod City. She married Matinee Idol Ricky Belmonte in 1970 and they separated in 1986. Their children are Sheryl, Renzo and Patrick.

Let us put to rest the maliscious notions on Senator Grace Poe's parentage. She is simply lucky enough to have been adopted and reared by the King and Queen of Philippine Movies. Some people have all the luck, indeed!