for me the reason why petunia kept harry is becasue of the letter dumbledore wrote...petunia having a sister as a witch must of heard dumbledores name more than we can imagine!!! although she says she hated her sister for being a witch there is still trust petunia gives for her sister...she must know that dumbledore is a great and powerful wizard and she agrees to kep him...i think i am teriibly wrong but oh well thats my opinion...we'll just have to find out in books six and seven why petunia accepted harry!!!