Well....then you be the hardest and meanest aunt of all time, so when they get back to your brother aka their father, theyll be like "Oh my, father, we have missed you so much, we will do all you say..as its so much better than your sisters"

You know..like the reverse of what I originally said to do...or...well, something like that, it is helping your little bro out. =)

Or of course, you just spoil em like crazy anyhow, its your..first two nieces? Might as well, as nieces only come along..so many in a lifetime, might as well be best friends with em.