Drew Family from PEI to Portland - c.1898

Looking for information on the Drew family that immigrated to the Portland Maine area before 1900. My grandmother was part of this group of 2 brothers and 4 sisters (plus her) that came from PEI.  I have found census records from 1900 and work records, for the older girls, from 1899.  One brother (Harry D Drew) and 3 sisters (Lulu Mooney (Drew), Myra Crandon (Drew) and Bertha Allen (Drew)) are accounted for via census, marriage and death records.  The other two brothers (John and Charles) and sister Caroline, I cannot find any records.  The huge question looming here is why did my grandmother move to the United States with her siblings, no mention of parents in the US (I have found an 1891 census from Canada listing the whole family)?  If anyone can assist with the answer to this question, I would appreciate it very much.