B.K.
Are the cousins sisters? If so I think I would pay them both the same. (If they weren't sisters I might pay the younger one a little less.) I pay $10 an hour. Seems to be the going rate around here. Either that or minimum wage, which is $8 now I think.
I have always paid my babysitters a little more than what they get elsewhere so that they will babysit for me first. :-) However, it's been getting harder to do that because most people I know pay $10 and I really don't want to try to one-up that!