I don't know where your Alexander and Agnes are buried, but both of the St-J-B candidates I mentioned in my earlier post are baptized, married and buried at St-J-B. The older, Pascal Alexis, s/o Charles and Soulange LEMAY, does, indeed, have a sister named Agnes:
1851 Census Index – Malden Twp. – page 1
Line 21 – REAUME, Charles – 49
Line 22 – REAUME, Mary – 42
Line 23 – REAUME, Pascal – 22 (candidate No. 1)
Line 24 – REAUME, Charles – 13
Line 25 – REAUME, Thomas – 10
Line 26 – REAUME, Mary
Line 27 – REAUME, Therese – 17
Line 28 – REAUME, Catherine – 16
Line 29 – REAUME, Agnes – 14
Line 30 – REAUME, Hariett – 9
Line 31 – REAUME, Ann – 13
Line 34 – REAUME, Antoine – 51
Line 35 – REAUME, Felice – 32
Line 36 – REAUME, Gilbert – 21
Line 37 - REAUME, Antoine – 19 (candidate No.2)
Line 38 – REAUME, Sophia – 15
Line 39 – REAUME, Daniel – 14
Line 40 – REAUME, Joseph - 12
Line 41 – REAUME, Fanny – 10
Line 42 – REAUME, Mary - 8
Line 43 – REAUME, Archange - 51
Line 44 – REAUME, Denis - 2
Line 48 – REAUME, Cecile – 72
Marriages – St-Jean-Baptiste, Amherstburg
REAUME, Pascal Alexis, bapt. 17 Jan 1830, s/o Charles and Soulange LEMAY, m. 9 Oct 1871, PROVENCHER, Marie, d/o Michel and Euphronsine PRUDHOMME
REAUME, Charles, s/o Charles and Soulange LEMAY, m. 24 Nov 1862, GOSTAGE, Joséphine, d/o John and Margaret HEALY
REAUME, Alex. Ant., s/o Antoine and Félice LEMAY, m. 8 Jan 1856, BOUCHER, Angélique
REAUME, Alex. Ant, 25, s/o Antoine and Félice LEMAY, widower of Angélique BOUCHER, m. BASTEIN, Angélique, d/o Augustin and Catherine RENAUD.
Burials – St-Jean-Baptiste, Amherstburg
REAUME, Alex. Ant. - s/o Antoine and Félice LEMAY, husband of (1) Angélique BOUCHER, (2) Angélique BASTEIN, bur. 3 Mar 1904, age 72
REAUME, Pascal Alexis – s/o Charles and Soulanges LEMAY, husband of Marie PROVENCHER, d. 15 Aug 1916, bur. 28 Aug 1913, age 87
There is an undocumented entry at familysearch.org that list Alexander Reaume, s/o Antoine and Felice LEMAY, born 1834, bapt. 27 Apr 1834, Amherstburg. I didn't think that matched the actual St-J-B record, though, which I thought gave this person a later birth/baptism. I can double-check.