Unless you have access to specific family bible records or public documents that specifically state otherwise, I think it would be prefectly logical to assume that your Mary Ann was actually named Mary Amanda. People in that day and age routinely switched their given and middle names around so this is not a surprising development.
Here is the birth record for Amanda C. Starkweather Knowles' daughter, Eva M. Knowles.
Michigan Births 1867-1902 Name: Eva M. Knowles Birth date: 28 Feb 1875 Birth place: Waverly, Van Buren, Michigan Gender: Female Race or color (on document): W Father name: Leavariah U. Knowles Father birth place: Michigan Age of father: Mother name: Amanda Knowles Mother birth place: New York Mother age: Christening date: Christening place: Additional relatives: Death date: Age at death: Film number: 2320526 Digital GS number: 4206447 Image number: 372 Frame number: Reference number: item 2 p 325 rn 36
So far, nothing else has come up for Eva. I have not yet found a birth record for Eva's sister, Ina Knowles, but I have a marriage record that is almost certainly hers. However, according to what I am seeing on the 1880 USC, Ina's year of birth on the marriage record is slightly "off". The discrepancy could be the result of a transcription error on the 1880 Census or else she gave an incorrect age.
Michigan Marriages 1868-1925 Groom name: Frank L. Youngs Groom race or color (on document): Groom age: 21 years Groom birth year: 1874 Groom birth place: Michigan Bride name: Ina V. Knowles Bride race or color (on document): Bride age: 20 years Bride birth year: 1875 Bride birth place: Michigan Marriage type: Marriage date: 03 Oct 1895 Marriage place: Paw Paw, Van Buren, Michigan Father of groom name: Lyman L. Youngs Mother of groom name: Belle Secord Father of bride name: L.V. Knowles Mother of bride name: Marital status: Groom previous wife name: Bride marital status: Bride previous husband name: Additional relatives: Film number: 2342502 Frame number: Digital GS number: 4207882 Image number: 38 Reference number: v 4 p 343 rn 2150
The 1880 Census record for the Knowles family reads:
1880 US Census for Waverly Twp., VanBuren Co., MI Family History Library Film: 1254608 NA Film Number: T9-0608 Page Number: 618C
Levenier M. KNOWLESSelfMMaleW34MIFarmerNYMI Amanda C. KNOWLESWifeMFemaleW34NYKeeping HouseNYNY Eva May KNOWLESDauSFemaleW6MIMINY Ina V. KNOWLESDauSFemaleW3MIMINY Elvin R. BLUEROtherSMaleW23MILaborerMINY Minnie HENRYOtherSFemaleW19MIAt Home......