It seems young to me to leave an 8 year old under a 10 year old's responsibility, but I think it depends on how responsible your nephews are. I did leave my daughter home for brief periods when she was 10 and 11 though she is an only child and didn't have a younger sibling to watch. Once she was 12 and up, I left her alone for longer periods.

I am pretty sure my mom left me home alone when I was in 4th grade, so I was probably 10 then too. She didn't leave my younger sisters home with me until I was 11 or 12. I was a pretty responsible, follow-the-rules type kid, and so were my sisters.